Preserve alpha channel of 1.0 when emulating RGB back buffer.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 214 matching lines @@
            GetMultisampledRenderbufferFormat() == GL_RGBA8_OES;
   }
 
   bool rgb_emulation =
       ContextProvider()->GetCapabilities().emulate_rgb_buffer_with_rgba ||
       (ShouldUseChromiumImage() &&
        ContextProvider()->GetCapabilities().chromium_image_rgb_emulation);
   return !want_alpha_channel_ && rgb_emulation;
 }
 
+void DrawingBuffer::MarkBlitFramebufferUsedThisFrame() {
+  blit_framebuffer_used_this_frame_ = true;
+}
+
 std::unique_ptr<cc::SharedBitmap> DrawingBuffer::CreateOrRecycleBitmap() {
   auto it = std::remove_if(
       recycled_bitmaps_.begin(), recycled_bitmaps_.end(),
       [this](const RecycledBitmap& bitmap) { return bitmap.size != size_; });
   recycled_bitmaps_.Shrink(it - recycled_bitmaps_.begin());
 
   if (!recycled_bitmaps_.IsEmpty()) {
     RecycledBitmap recycled = std::move(recycled_bitmaps_.back());
     recycled_bitmaps_.pop_back();
     DCHECK(recycled.size == size_);
@@ skipping 161 matching lines @@
         WTF::Bind(&DrawingBuffer::MailboxReleasedGpu,
                   RefPtr<DrawingBuffer>(this), color_buffer_for_mailbox);
     *out_release_callback = cc::SingleReleaseCallback::Create(
         ConvertToBaseCallback(std::move(func)));
   }
 
   // Point |m_frontColorBuffer| to the buffer that we are now presenting.
   front_color_buffer_ = color_buffer_for_mailbox;
 
   contents_changed_ = false;
+  blit_framebuffer_used_this_frame_ = false;
   SetBufferClearNeeded(true);
   return true;
 }
 
 void DrawingBuffer::MailboxReleasedGpu(RefPtr<ColorBuffer> color_buffer,
                                        const gpu::SyncToken& sync_token,
                                        bool lost_resource) {
   // If the mailbox has been returned by the compositor then it is no
   // longer being presented, and so is no longer the front buffer.
   if (color_buffer == front_color_buffer_)
@@ skipping 586 matching lines @@
   }
 
   gl_->BindFramebuffer(GL_FRAMEBUFFER, fbo_);
   if (anti_aliasing_mode_ == kScreenSpaceAntialiasing)
     gl_->ApplyScreenSpaceAntialiasingCHROMIUM();
 }
 
 void DrawingBuffer::ResolveIfNeeded() {
   if (anti_aliasing_mode_ != kNone)
     ResolveMultisampleFramebufferInternal();
+
+  // If the user performed a manual BlitFramebuffer to the default
+  // back buffer, they may have populated the alpha channel with
+  // values not equal to 1.0. Must undo this.
+  if (DefaultBufferRequiresAlphaChannelToBePreserved() &&
+      blit_framebuffer_used_this_frame_) {
+    // Avoid repeating this work if it's already been done.
+    if (anti_aliasing_mode_ != kNone &&
+        ContextProvider()
+            ->GetCapabilities()
+            .disable_multisampling_color_mask_usage) {
+      // Do nothing.
+    } else {
+      DCHECK(state_restorer_);
+      state_restorer_->SetClearStateDirty();
+      gl_->ClearColor(0, 0, 0, 1);
+      gl_->ColorMask(false, false, false, true);
+      gl_->Clear(GL_COLOR_BUFFER_BIT);
+    }
+  }
+
   contents_change_resolved_ = true;
 }
 
 void DrawingBuffer::RestoreFramebufferBindings() {
   client_->DrawingBufferClientRestoreFramebufferBinding();
 }
 
 void DrawingBuffer::RestoreAllState() {
   client_->DrawingBufferClientRestoreScissorTest();
   client_->DrawingBufferClientRestoreMaskAndClearValues();
@@ skipping 269 matching lines @@
   if (pixel_unpack_buffer_binding_dirty_)
     client->DrawingBufferClientRestorePixelUnpackBufferBinding();
 }
 
 bool DrawingBuffer::ShouldUseChromiumImage() {
   return RuntimeEnabledFeatures::webGLImageChromiumEnabled() &&
          chromium_image_usage_ == kAllowChromiumImage;
 }
 
 }  // namespace blink