[devtools] Allow transparent base background color overrides.

third_party/WebKit/Source/web/WebViewImpl.h

Index: third_party/WebKit/Source/web/WebViewImpl.h
diff --git a/third_party/WebKit/Source/web/WebViewImpl.h b/third_party/WebKit/Source/web/WebViewImpl.h
index 506b24524b9b3ca8ccaf19af3a0a41788c95d448..7e58d495f716a161fa97216cd57c0a91308b4abc 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.h
+++ b/third_party/WebKit/Source/web/WebViewImpl.h
@@ -286,6 +286,7 @@ class WEB_EXPORT WebViewImpl final
 
   void setBaseBackgroundColor(WebColor);
   void setBaseBackgroundColorOverride(WebColor);
+  void clearBaseBackgroundColorOverride();
   void setBackgroundColorOverride(WebColor);
   void setZoomFactorOverride(float);
   void setCompositorDeviceScaleFactorOverride(float);
@@ -706,6 +707,7 @@ class WEB_EXPORT WebViewImpl final
   std::unique_ptr<FullscreenController> m_fullscreenController;
 
   WebColor m_baseBackgroundColor;
+  bool m_baseBackgroundColorOverrideEnabled;
   WebColor m_baseBackgroundColorOverride;
   WebColor m_backgroundColorOverride;
   float m_zoomFactorOverride;