Fix ref-cnting bug in GrContext::readRenderTargetPixels
Fix ref-cnting bug in GrContext::readRenderTargetPixels

TBR=egdaniel@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/732713002

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LovesYou/skia-sys,Jichao/skia,chenlian2015/skia_from_google,qrealka/skia-hc,BrokenROM/external_skia,TeamTwisted/external_skia,Hikari-no-Tenshi/android_external_skia,boulzordev/android_external_skia,chenlian2015/skia_from_google,ominux/skia,timduru/platform-external-skia,nfxosp/platform_external_skia,todotodoo/skia,MonkeyZZZZ/platform_external_skia,boulzordev/android_external_skia,pcwalton/skia,Infinitive-OS/platform_external_skia,BrokenROM/external_skia,MonkeyZZZZ/platform_external_skia,MonkeyZZZZ/platform_external_skia,Igalia/skia,ominux/skia,amyvmiwei/skia,Hikari-no-Tenshi/android_external_skia,MarshedOut/android_external_skia,YUPlayGodDev/platform_external_skia,ominux/skia,AOSPB/external_skia,PAC-ROM/android_external_skia,YUPlayGodDev/platform_external_skia,pcwalton/skia,DiamondLovesYou/skia-sys,TeamTwisted/external_skia,DiamondLovesYou/skia-sys,MarshedOut/android_external_skia,DiamondLovesYou/skia-sys,VRToxin-AOSP/android_external_skia,BrokenROM/external_skia,tmpvar/skia.cc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,ominux/skia,w3nd1go/android_external_skia,google/skia,tmpvar/skia.cc,amyvmiwei/skia,tmpvar/skia.cc,AOSP-YU/platform_external_skia,todotodoo/skia,nfxosp/platform_external_skia,google/skia,todotodoo/skia,timduru/platform-external-skia,boulzordev/android_external_skia,OneRom/external_skia,geekboxzone/mmallow_external_skia,TeamTwisted/external_skia,timduru/platform-external-skia,Infinitive-OS/platform_external_skia,OneRom/external_skia,PAC-ROM/android_external_skia,ominux/skia,rubenvb/skia,chenlian2015/skia_from_google,samuelig/skia,google/skia,OneRom/external_skia,tmpvar/skia.cc,timduru/platform-external-skia,AOSPB/external_skia,amyvmiwei/skia,vanish87/skia,Hikari-no-Tenshi/android_external_skia,MarshedOut/android_external_skia,scroggo/skia,todotodoo/skia,TeamExodus/external_skia,PAC-ROM/android_external_skia,invisiblek/android_external_skia,VRToxin-AOSP/android_external_skia,PAC-ROM/android_external_skia,MinimalOS-AOSP/platform_external_skia,rubenvb/skia,todotodoo/skia,AOSPB/external_skia,Jichao/skia,samuelig/skia,nvoron23/skia,TeamExodus/external_skia,ominux/skia,Jichao/skia,TeamTwisted/external_skia,noselhq/skia,invisiblek/android_external_skia,AOSP-YU/platform_external_skia,nfxosp/platform_external_skia,UBERMALLOW/external_skia,pcwalton/skia,Infinitive-OS/platform_external_skia,OneRom/external_skia,TeamTwisted/external_skia,todotodoo/skia,scroggo/skia,TeamExodus/external_skia,Hikari-no-Tenshi/android_external_skia,scroggo/skia,AOSPB/external_skia,jtg-gg/skia,Igalia/skia,geekboxzone/mmallow_external_skia,vanish87/skia,nvoron23/skia,samuelig/skia,TeamExodus/external_skia,aosp-mirror/platform_external_skia,Jichao/skia,nvoron23/skia,Hikari-no-Tenshi/android_external_skia,Jichao/skia,scroggo/skia,qrealka/skia-hc,rubenvb/skia,TeamExodus/external_skia,nfxosp/platform_external_skia,spezi77/android_external_skia,MarshedOut/android_external_skia,MarshedOut/android_external_skia,aosp-mirror/platform_external_skia,Jichao/skia,PAC-ROM/android_external_skia,AOSP-YU/platform_external_skia,MarshedOut/android_external_skia,OneRom/external_skia,MonkeyZZZZ/platform_external_skia,pcwalton/skia,TeamExodus/external_skia,shahrzadmn/skia,scroggo/skia,Jichao/skia,AOSP-YU/platform_external_skia,VRToxin-AOSP/android_external_skia,w3nd1go/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,nfxosp/platform_external_skia,nvoron23/skia,aosp-mirror/platform_external_skia,VRToxin-AOSP/android_external_skia,TeamExodus/external_skia,nvoron23/skia,geekboxzone/mmallow_external_skia,boulzordev/android_external_skia,aosp-mirror/platform_external_skia,UBERMALLOW/external_skia,TeamTwisted/external_skia,rubenvb/skia,MinimalOS-AOSP/platform_external_skia,shahrzadmn/skia