Cleanup: Delete SkFlattenableBuffers header.
Cleanup: Delete SkFlattenableBuffers header.

It is empty. cs.chromium.org shows no one is including it.

BUG=skia:3108
R=reed@google.com

Review URL: https://codereview.chromium.org/722943002

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