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

third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp

Index: third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp b/third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp
index 4477ed4c6197b6129c2ae4c7fd395bfc927ba4c4..e3574539fed08809a9046c7ddcc998701d4ffff6 100644
--- a/third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp
+++ b/third_party/WebKit/Source/web/WebFrameWidgetImpl.cpp
@@ -267,7 +267,10 @@ 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() {
@@ -280,9 +283,7 @@ void WebFrameWidgetImpl::updateLayerTreeDeviceScaleFactor() {
 
 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 {