Blend background with existing content

Source/core/page/Settings.in

 # Defines properties which are available on the Settings object.
 #
 # Please think carefully before adding a new Setting.  Some questions to
 # consider are:
 # - Should this be a RuntimeEnabledFeature instead? Settings are for things
 #   which we support either values of at runtime.  Features are set at renderer
 #   process startup and are never changed. Features also tend to be set to a
 #   value based on the platform or the stability of the code in question, where
 #   as settings both codepaths need to be stable.
 # - How will you ensure test coverage of all relevant values of your setting?
@@ skipping 52 matching lines @@
 editingBehaviorType type=EditingBehaviorType, initial=editingBehaviorTypeForPlatform()
 
 caretBrowsingEnabled initial=false
 localStorageEnabled initial=false
 allowUniversalAccessFromFileURLs initial=true
 allowFileAccessFromFileURLs initial=true
 javaScriptCanOpenWindowsAutomatically initial=false
 supportsMultipleWindows initial=true
 javaScriptCanAccessClipboard initial=false
 shouldPrintBackgrounds initial=false
+shouldClearDocumentBackground initial=true
 
 textAreasAreResizable initial=false, setNeedsStyleRecalcInAllFrames=1
 acceleratedCompositingEnabled initial=true, setNeedsStyleRecalcInAllFrames=1
 
 # Debugging feature used for accelerated compositing layers.
 showRepaintCounter initial=false, setNeedsStyleRecalcInAllFrames=1
 
 shrinksStandaloneImagesToFit initial=true
 
 # FIXME: Does this do anything now that we don't support page cache?
@@ skipping 159 matching lines @@
 # behaves.  TODO(wjmaclean): link to design document.
 # crbug.com/304869 tracks removal.
 pinchVirtualViewportEnabled initial=false
 useSolidColorScrollbars initial=false
 
 mainFrameClipsContent initial=true
 
 # Enable decoration shadow nodes to test password genration feature
 # crbug.com/114092. This flag should not be enabled for production.
 passwordGenerationDecorationEnabled initial=false