[blink] Support (semi-)transparent background colors in WebView/Frame.

third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp

 /*
  * Copyright (C) 2014 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 249 matching lines @@
   // Pass the limits even though this is for subframes, as the limits will be
   // needed in setting the raster scale.
   m_layerTreeView->setPageScaleFactorAndLimits(
       1, view()->minimumPageScaleFactor(), view()->maximumPageScaleFactor());
 }
 
 void WebFrameWidgetImpl::updateLayerTreeBackgroundColor() {
   if (!m_layerTreeView)
     return;
 
-  m_layerTreeView->setBackgroundColor(backgroundColor());
+  WebColor color = backgroundColor();
+  m_layerTreeView->setBackgroundColor(color);
+  m_layerTreeView->setHasTransparentBackground(alphaChannel(color) <
+                                               SK_AlphaOPAQUE);
 }
 
 void WebFrameWidgetImpl::updateLayerTreeDeviceScaleFactor() {
   DCHECK(page());
   DCHECK(m_layerTreeView);
 
   float deviceScaleFactor = page()->deviceScaleFactorDeprecated();
   m_layerTreeView->setDeviceScaleFactor(deviceScaleFactor);
 }
 
 void WebFrameWidgetImpl::setIsTransparent(bool isTransparent) {
   m_isTransparent = isTransparent;

[chrishtr, 2017/03/01 16:16:53]

It seems it is not guaranteed that m_isTransparent and
alphaChannel(color) < SK_AlphaOPAQUE are the same.

Should there even be a setIsTransparent method if we always
know the actual background color including transparency?

[Eric Seckler, 2017/03/07 18:00:47]

FWIW, I think this would have been sort-of guaranteed because m_isTransparent
overrode the backgroundColor to transparent.
Since lfg@ suggested the same, I'm attempting to get rid of it! :)

-
-  if (m_layerTreeView)
-    m_layerTreeView->setHasTransparentBackground(isTransparent);
+  updateLayerTreeBackgroundColor();
 }
 
 bool WebFrameWidgetImpl::isTransparent() const {
   return m_isTransparent;
 }
 
 void WebFrameWidgetImpl::layoutAndPaintAsync(
     WebLayoutAndPaintAsyncCallback* callback) {
   m_layerTreeView->layoutAndPaintAsync(callback);
 }
@@ skipping 846 matching lines @@
   return nullptr;
 }
 
 LocalFrame* WebFrameWidgetImpl::focusedLocalFrameAvailableForIme() const {
   if (!m_imeAcceptEvents)
     return nullptr;
   return focusedLocalFrameInWidget();
 }
 
 }  // namespace blink