Prevent implicit framebuffer clear from clobbering alpha in emulated RGB

third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h

Index: third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h
index a7b105741259c5e6c2394d39e2cb4c2884ff18e0..6666867c4ed6ec327e7504bb899494e711e98629 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h
+++ b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h
@@ -111,13 +111,13 @@ class WebGLRenderingContextErrorMessageCallback;
 // the DrawingBuffer requires RGB emulation.
 class ScopedRGBEmulationColorMask {
  public:
-  ScopedRGBEmulationColorMask(gpu::gles2::GLES2Interface*,
+  ScopedRGBEmulationColorMask(WebGLRenderingContextBase*,
                               GLboolean* colorMask,
                               DrawingBuffer*);
   ~ScopedRGBEmulationColorMask();
 
  private:
-  gpu::gles2::GLES2Interface* m_contextGL;
+  WeakPersistent<WebGLRenderingContextBase> m_context;

[Ken Russell (switch to Gerrit), 2016/12/01 07:20:43]

I'm concerned about the performance impact of allocating a WeakPersistent during
every draw call. haraken@, sigbjornf@, could you comment on that? These data
structures are only stack-allocated. Is there some cheaper way to be safe with
regard to Oilpan but still maintain temporary pointers to the
WebGLRenderingContextBase? With Oilpan's clang plugin, we found this necessary
to compile.

[Ken Russell (switch to Gerrit), 2016/12/01 08:36:47]

I just learned from looking at another CL (
https://codereview.chromium.org/2476693002/ ) that the way to handle this is to
declare this STACK_ALLOCATED(), and use a Member. Kai, could you make that
change?

Kentaro, Sigbjorn, no longer necessary to look at this.

[Kai Ninomiya, 2016/12/01 23:30:20]

Done.

   GLboolean m_colorMask[4];
   const bool m_requiresEmulation;
 };
@@ -592,6 +592,16 @@ class MODULES_EXPORT WebGLRenderingContextBase : public CanvasRenderingContext,
 
   void commit(ScriptState*, ExceptionState&);
 
+  std::list<ScopedRGBEmulationColorMask*> activeScopedRGBEmulationColorMasks;

[Ken Russell (switch to Gerrit), 2016/12/01 07:20:43]

Please put all of this in the private section and make
ScopedRGBEmulationColorMask a friend.

[Kai Ninomiya, 2016/12/01 23:30:20]

Done.

+  void pushActiveScopedRGBEmulationColorMask(ScopedRGBEmulationColorMask* val) {
+    activeScopedRGBEmulationColorMasks.push_back(val);
+  }
+  ScopedRGBEmulationColorMask* popActiveScopedRGBEmulationColorMask() {
+    auto removed = activeScopedRGBEmulationColorMasks.back();
+    activeScopedRGBEmulationColorMasks.pop_back();
+    return removed;
+  }
+
  protected:
   friend class EXTDisjointTimerQuery;
   friend class EXTDisjointTimerQueryWebGL2;