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..c8551e69827a7d782d96d0e00a9ad6ed416e8511 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h
+++ b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.h
@@ -110,14 +110,16 @@ class WebGLRenderingContextErrorMessageCallback;
 // This class uses the color mask to prevent drawing to the alpha channel, if
 // the DrawingBuffer requires RGB emulation.
 class ScopedRGBEmulationColorMask {
+  STACK_ALLOCATED();
+
  public:
-  ScopedRGBEmulationColorMask(gpu::gles2::GLES2Interface*,
+  ScopedRGBEmulationColorMask(WebGLRenderingContextBase*,
                               GLboolean* colorMask,
                               DrawingBuffer*);
   ~ScopedRGBEmulationColorMask();
 
  private:
-  gpu::gles2::GLES2Interface* m_contextGL;
+  Member<WebGLRenderingContextBase> m_context;
   GLboolean m_colorMask[4];
   const bool m_requiresEmulation;
 };
@@ -1533,6 +1535,17 @@ class MODULES_EXPORT WebGLRenderingContextBase : public CanvasRenderingContext,
   static WebGLRenderingContextBase* oldestContext();
   static WebGLRenderingContextBase* oldestEvictedContext();
 
+  friend class ScopedRGBEmulationColorMask;
+  std::list<ScopedRGBEmulationColorMask*> activeScopedRGBEmulationColorMasks;
+  void pushActiveScopedRGBEmulationColorMask(ScopedRGBEmulationColorMask* val) {
+    activeScopedRGBEmulationColorMasks.push_back(val);
+  }
+  ScopedRGBEmulationColorMask* popActiveScopedRGBEmulationColorMask() {

[Ken Russell (switch to Gerrit), 2016/12/01 23:45:30]

It occurs to me that since the WebGLRenderingContextBase code only checks
activeScopedRGBEmulationColorMasks.empty(), we could just use a simple int to
track the active count instead of a std::list. I'd ask that this change be made
for simplicity.

[Kai Ninomiya, 2016/12/02 00:05:27]

Great point, thanks. I was thinking of trying to simplify it from a std::list
but I forgot to figure out if I could.

Done.

+    auto removed = activeScopedRGBEmulationColorMasks.back();
+    activeScopedRGBEmulationColorMasks.pop_back();
+    return removed;
+  }
+
   ImageBitmap* transferToImageBitmapBase(ScriptState*);
 
   // Helper functions for tex(Sub)Image2D && texSubImage3D