Sets a transparent background for out-of-process subframes.

third_party/WebKit/public/web/WebFrameWidget.h

 /*
  * 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 37 matching lines @@
     // away from WebView inheriting WebWidget.
     // TODO(dcheng): Remove once transition is complete.
     BLINK_EXPORT static WebFrameWidget* create(WebWidgetClient*, WebView*, WebLocalFrame* mainFrame);
 
     // Sets the visibility of the WebFrameWidget.
     // We still track page-level visibility, but additionally we need to notify a WebFrameWidget
     // when its owning RenderWidget receives a Show or Hide directive, so that it knows whether
     // it needs to draw or not.
     virtual void setVisibilityState(WebPageVisibilityState visibilityState, bool isInitialState) { }
 
+    // Makes the WebFrameWidget transparent.  This is useful if you want to have
+    // some custom background rendered behind it.
+    virtual bool isTransparent() const = 0;
+    virtual void setIsTransparent(bool) = 0;
+
+    // Sets the base color used for this WebFrameWidget's background. This is in
+    // effect the default background color used for pages with no
+    // background-color style in effect, or used as the alpha-blended basis for
+    // any pages with translucent background-color style. (For pages with opaque
+    // background-color style, this property is effectively ignored).
+    // Setting this takes effect for the currently loaded page, if any, and
+    // persists across subsequent navigations. Defaults to white prior to the
+    // first call to this method.
+    virtual void setBaseBackgroundColor(WebColor) = 0;
+
+    // Overrides the background color for this WebFrameWidget.
+    virtual void setBackgroundColorOverride(WebColor) = 0;

[dcheng, 2016/02/10 22:13:44]

Does this need to be the in public layer? I don't actually see any calls to this
method through a WebFrameWidget pointer.

The only calls I see to this are in DevToolsEmulator, which lives in Source/web.
So how does this get set? What am I missing?

[lfg, 2016/02/10 23:20:24]

If I remember correctly, when I did this I was planning on converting all calls
to WebView to calls to WebFrameWidget, but ended up forgetting to do it. In that
case we would need this public interface, at least temporarily until
WebFrameWidget/WebViewFrameWidget is disentangled.

I could either remove the public interfaces, convert the call sites to use
WebFrameWidget, or convert the call sites and assume the widgets are
WebViewFrameWidgets. What do you think we should do for now?

+    virtual WebColor backgroundColorOverride() const = 0;
+
     // TODO(dcheng): Temporary: there should only be one WebFrameWidget
     // implementation but the Blink API is currently in a transition state.
     // See https://goo.gl/7yVrnb. These methods should only be used inside
     // blink.
     virtual bool forSubframe() const { return true; }
     virtual void scheduleAnimation() = 0;
 };
 
 } // namespace blink
 
 #endif