Detect discarded SkPictureShader pixel refs.
Detect discarded SkPictureShader pixel refs.

This doesn't address the more general problem, but reduces the race
window significantly.

BUG=440282
R=reed@google.com

Review URL: https://codereview.chromium.org/787043006

rubenvb/skia,todotodoo/skia,shahrzadmn/skia,vanish87/skia,noselhq/skia,TeamExodus/external_skia,TeamTwisted/external_skia,OneRom/external_skia,AOSP-YU/platform_external_skia,rubenvb/skia,Igalia/skia,PAC-ROM/android_external_skia,tmpvar/skia.cc,HalCanary/skia-hc,Infinitive-OS/platform_external_skia,w3nd1go/android_external_skia,shahrzadmn/skia,geekboxzone/mmallow_external_skia,ominux/skia,vanish87/skia,nfxosp/platform_external_skia,scroggo/skia,MonkeyZZZZ/platform_external_skia,YUPlayGodDev/platform_external_skia,YUPlayGodDev/platform_external_skia,MarshedOut/android_external_skia,aosp-mirror/platform_external_skia,Jichao/skia,shahrzadmn/skia,UBERMALLOW/external_skia,w3nd1go/android_external_skia,spezi77/android_external_skia,MarshedOut/android_external_skia,Infinitive-OS/platform_external_skia,geekboxzone/mmallow_external_skia,Jichao/skia,timduru/platform-external-skia,Jichao/skia,timduru/platform-external-skia,rubenvb/skia,w3nd1go/android_external_skia,PAC-ROM/android_external_skia,tmpvar/skia.cc,boulzordev/android_external_skia,google/skia,aosp-mirror/platform_external_skia,MonkeyZZZZ/platform_external_skia,amyvmiwei/skia,amyvmiwei/skia,TeamExodus/external_skia,AOSPB/external_skia,aosp-mirror/platform_external_skia,AOSPB/external_skia,pcwalton/skia,todotodoo/skia,rubenvb/skia,nfxosp/platform_external_skia,Infinitive-OS/platform_external_skia,UBERMALLOW/external_skia,amyvmiwei/skia,todotodoo/skia,Infinitive-OS/platform_external_skia,HalCanary/skia-hc,noselhq/skia,Jichao/skia,MinimalOS-AOSP/platform_external_skia,scroggo/skia,BrokenROM/external_skia,BrokenROM/external_skia,Infinitive-OS/platform_external_skia,VRToxin-AOSP/android_external_skia,todotodoo/skia,nfxosp/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,spezi77/android_external_skia,TeamExodus/external_skia,geekboxzone/mmallow_external_skia,ominux/skia,PAC-ROM/android_external_skia,boulzordev/android_external_skia,HalCanary/skia-hc,TeamTwisted/external_skia,todotodoo/skia,ominux/skia,OneRom/external_skia,Igalia/skia,AOSP-YU/platform_external_skia,w3nd1go/android_external_skia,noselhq/skia,MinimalOS-AOSP/platform_external_skia,DiamondLovesYou/skia-sys,geekboxzone/mmallow_external_skia,tmpvar/skia.cc,scroggo/skia,nvoron23/skia,AOSPB/external_skia,vanish87/skia,MarshedOut/android_external_skia,MonkeyZZZZ/platform_external_skia,w3nd1go/android_external_skia,vanish87/skia,Hikari-no-Tenshi/android_external_skia,scroggo/skia,UBERMALLOW/external_skia,Hikari-no-Tenshi/android_external_skia,nvoron23/skia,invisiblek/android_external_skia,tmpvar/skia.cc,YUPlayGodDev/platform_external_skia,geekboxzone/mmallow_external_skia,qrealka/skia-hc,ominux/skia,AOSP-YU/platform_external_skia,spezi77/android_external_skia,nvoron23/skia,shahrzadmn/skia,samuelig/skia,OneRom/external_skia,MarshedOut/android_external_skia,VRToxin-AOSP/android_external_skia,todotodoo/skia,scroggo/skia,MarshedOut/android_external_skia,MarshedOut/android_external_skia,Jichao/skia,tmpvar/skia.cc,vanish87/skia,amyvmiwei/skia,boulzordev/android_external_skia,pcwalton/skia,pcwalton/skia,TeamTwisted/external_skia,OneRom/external_skia,TeamTwisted/external_skia,samuelig/skia,PAC-ROM/android_external_skia,MonkeyZZZZ/platform_external_skia,BrokenROM/external_skia,Hikari-no-Tenshi/android_external_skia,AOSPB/external_skia,tmpvar/skia.cc,boulzordev/android_external_skia,invisiblek/android_external_skia,qrealka/skia-hc,qrealka/skia-hc,HalCanary/skia-hc,noselhq/skia,UBERMALLOW/external_skia,BrokenROM/external_skia,samuelig/skia,qrealka/skia-hc,TeamExodus/external_skia,jtg-gg/skia,tmpvar/skia.cc,YUPlayGodDev/platform_external_skia,nfxosp/platform_external_skia,YUPlayGodDev/platform_external_skia,shahrzadmn/skia,noselhq/skia,MinimalOS-AOSP/platform_external_skia,TeamExodus/external_skia,UBERMALLOW/external_skia,PAC-ROM/android_external_skia,MonkeyZZZZ/platform_external_skia,google/skia,TeamExodus/external_skia,Hikari-no-Tenshi/android_external_skia,Jichao/skia,Igalia/skia,boulzordev/android_external_skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,noselhq/skia,AOSPB/external_skia,timduru/platform-external-skia,invisiblek/android_external_skia,MinimalOS-AOSP/platform_external_skia,google/skia,boulzordev/android_external_skia,UBERMALLOW/external_skia,Igalia/skia,PAC-ROM/android_external_skia,geekboxzone/mmallow_external_skia,aosp-mirror/platform_external_skia,jtg-gg/skia,ominux/skia,amyvmiwei/skia,samuelig/skia,shahrzadmn/skia,geekboxzone/mmallow_external_skia,jtg-gg/skia,MarshedOut/android_external_skia,OneRom/external_skia,nvoron23/skia,ominux/skia,DiamondLovesYou/skia-sys,rubenvb/skia,nvoron23/skia,amyvmiwei/skia,rubenvb/skia,AOSP-YU/platform_external_skia,rubenvb/skia,boulzordev/android_external_skia,Jichao/skia,timduru/platform-external-skia,qrealka/skia-hc,timduru/platform-external-skia,HalCanary/skia-hc,TeamExodus/external_skia,google/skia,timduru/platform-external-skia,MonkeyZZZZ/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,jtg-gg/skia,BrokenROM/external_skia,invisiblek/android_external_skia,VRToxin-AOSP/android_external_skia,qrealka/skia-hc,ominux/skia,Hikari-no-Tenshi/android_external_skia,jtg-gg/skia,MonkeyZZZZ/platform_external_skia,BrokenROM/external_skia,TeamTwisted/external_skia,noselhq/skia,Hikari-no-Tenshi/android_external_skia,w3nd1go/android_external_skia,google/skia,VRToxin-AOSP/android_external_skia,google/skia,geekboxzone/mmallow_external_skia,w3nd1go/android_external_skia,spezi77/android_external_skia,vanish87/skia,Jichao/skia,w3nd1go/android_external_skia,nfxosp/platform_external_skia,spezi77/android_external_skia,PAC-ROM/android_external_skia,scroggo/skia,MonkeyZZZZ/platform_external_skia,nvoron23/skia,shahrzadmn/skia,nfxosp/platform_external_skia,DiamondLovesYou/skia-sys,DiamondLovesYou/skia-sys,MinimalOS-AOSP/platform_external_skia,pcwalton/skia,MinimalOS-AOSP/platform_external_skia,MinimalOS-AOSP/platform_external_skia,YUPlayGodDev/platform_external_skia,google/skia,vanish87/skia,UBERMALLOW/external_skia,invisiblek/android_external_skia,google/skia,nvoron23/skia,rubenvb/skia,PAC-ROM/android_external_skia,qrealka/skia-hc,OneRom/external_skia,samuelig/skia,aosp-mirror/platform_external_skia,boulzordev/android_external_skia,TeamExodus/external_skia,MinimalOS-AOSP/platform_external_skia,google/skia,AOSP-YU/platform_external_skia,TeamTwisted/external_skia,HalCanary/skia-hc,Igalia/skia,ominux/skia,pcwalton/skia,Infinitive-OS/platform_external_skia,DiamondLovesYou/skia-sys,TeamTwisted/external_skia,Hikari-no-Tenshi/android_external_skia,BrokenROM/external_skia,DiamondLovesYou/skia-sys,qrealka/skia-hc,AOSP-YU/platform_external_skia,Hikari-no-Tenshi/android_external_skia,scroggo/skia,BrokenROM/external_skia,AOSPB/external_skia,samuelig/skia,rubenvb/skia,jtg-gg/skia,aosp-mirror/platform_external_skia,AOSP-YU/platform_external_skia,aosp-mirror/platform_external_skia,VRToxin-AOSP/android_external_skia,YUPlayGodDev/platform_external_skia,MarshedOut/android_external_skia,tmpvar/skia.cc,Infinitive-OS/platform_external_skia,AOSP-YU/platform_external_skia,ominux/skia,timduru/platform-external-skia,samuelig/skia,invisiblek/android_external_skia,shahrzadmn/skia,scroggo/skia,Igalia/skia,pcwalton/skia,UBERMALLOW/external_skia,boulzordev/android_external_skia,Infinitive-OS/platform_external_skia,shahrzadmn/skia,TeamExodus/external_skia,rubenvb/skia,samuelig/skia,AOSPB/external_skia,nvoron23/skia,TeamTwisted/external_skia,vanish87/skia,vanish87/skia,Jichao/skia,jtg-gg/skia,amyvmiwei/skia,nfxosp/platform_external_skia,todotodoo/skia,aosp-mirror/platform_external_skia,w3nd1go/android_external_skia,nfxosp/platform_external_skia,DiamondLovesYou/skia-sys,MinimalOS-AOSP/platform_external_skia,Igalia/skia,HalCanary/skia-hc,MarshedOut/android_external_skia,spezi77/android_external_skia,TeamTwisted/external_skia,pcwalton/skia,Igalia/skia,PAC-ROM/android_external_skia,VRToxin-AOSP/android_external_skia,nvoron23/skia,AOSPB/external_skia,YUPlayGodDev/platform_external_skia,noselhq/skia,pcwalton/skia,AOSP-YU/platform_external_skia,todotodoo/skia,invisiblek/android_external_skia,OneRom/external_skia,noselhq/skia,VRToxin-AOSP/android_external_skia,amyvmiwei/skia,pcwalton/skia,todotodoo/skia,VRToxin-AOSP/android_external_skia,OneRom/external_skia,invisiblek/android_external_skia,MonkeyZZZZ/platform_external_skia,OneRom/external_skia,YUPlayGodDev/platform_external_skia,UBERMALLOW/external_skia,AOSPB/external_skia,VRToxin-AOSP/android_external_skia,Infinitive-OS/platform_external_skia,nfxosp/platform_external_skia,geekboxzone/mmallow_external_skia,google/skia