Preserve alpha channel of 1.0 when emulating RGB back buffer.

third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp

Index: third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp
diff --git a/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp b/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp
index 3d554f9a4d72ac1c51e7d07e791a817fc4ce65d8..28962bf5e795f1333d31c31731e4f6c6955d500f 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp
+++ b/third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp
@@ -427,6 +427,14 @@ void WebGL2RenderingContextBase::blitFramebuffer(GLint src_x0,
 
   ContextGL()->BlitFramebufferCHROMIUM(src_x0, src_y0, src_x1, src_y1, dst_x0,
                                        dst_y0, dst_x1, dst_y1, mask, filter);
+
+  // If blitting from a user-defined framebuffer to the default back
+  // buffer, there is a possibility of overwriting the alpha channel
+  // in ways that can result in incorrect compositing. Notify the
+  // DrawingBuffer in this case.
+  if (read_framebuffer_binding_ && !framebuffer_binding_) {
+    GetDrawingBuffer()->MarkBlitFramebufferUsedThisFrame();
+  }
 }
 
 bool WebGL2RenderingContextBase::ValidateTexFuncLayer(const char* function_name,