Preserve alpha channel of 1.0 when emulating RGB back buffer.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/webgl/WebGL2RenderingContextBase.h"
 
 #include <memory>
 #include "bindings/modules/v8/WebGLAny.h"
 #include "core/frame/ImageBitmap.h"
 #include "core/html/HTMLCanvasElement.h"
@@ skipping 409 matching lines @@
                                                  GLint dst_y0,
                                                  GLint dst_x1,
                                                  GLint dst_y1,
                                                  GLbitfield mask,
                                                  GLenum filter) {
   if (isContextLost())
     return;
 
   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,
                                                       GLenum tex_target,
                                                       GLint layer) {
   if (layer < 0) {
     SynthesizeGLError(GL_INVALID_VALUE, function_name, "layer out of range");
     return false;
   }
   switch (tex_target) {
@@ skipping 5317 matching lines @@
 
 void WebGL2RenderingContextBase::
     DrawingBufferClientRestorePixelUnpackBufferBinding() {
   if (!ContextGL())
     return;
   ContextGL()->BindBuffer(GL_PIXEL_UNPACK_BUFFER,
                           ObjectOrZero(bound_pixel_unpack_buffer_.Get()));
 }
 
 }  // namespace blink