Remove the acceleratedCompositingForPluginsEnabled Setting.

Source/core/frame/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 77 matching lines @@
 # don't support the feature download files they can't use by.
 # Leaving enabled for now to not change existing behavior.
 downloadableBinaryFontsEnabled initial=true
 
 xssAuditorEnabled initial=false
 unsafePluginPastingEnabled initial=true
 treatIPAddressAsDomain initial=false
 
 acceleratedCompositingFor3DTransformsEnabled initial=true, invalidate=AcceleratedCompositing
 acceleratedCompositingForVideoEnabled initial=true, invalidate=AcceleratedCompositing
-acceleratedCompositingForPluginsEnabled initial=true, invalidate=AcceleratedCompositing
 acceleratedCompositingForCanvasEnabled initial=true, invalidate=AcceleratedCompositing
 acceleratedCompositingForAnimationEnabled initial=true, invalidate=AcceleratedCompositing
 acceleratedCompositingForFiltersEnabled initial=false, invalidate=AcceleratedCompositing
 acceleratedCompositingForFixedPositionEnabled initial=false, invalidate=AcceleratedCompositing
 acceleratedCompositingForOverflowScrollEnabled initial=false, invalidate=AcceleratedCompositing
 acceleratedCompositingForFixedRootBackgroundEnabled initial=false
 acceleratedCompositingForGpuRasterizationHintEnabled initial=false, invalidate=AcceleratedCompositing
 
 forceCompositingMode initial=false, invalidate=AcceleratedCompositing
 
@@ skipping 165 matching lines @@
 
 # Clients that execute script should call ScriptController::canExecuteScripts()
 # instead of this function. ScriptController::canExecuteScripts() checks the
 # HTML sandbox, plug-in sandboxing, and other important details.
 scriptEnabled initial=false, invalidate=ScriptEnable
 
 # Compensates for poor text legibility on mobile devices. This value is
 # multiplied by the font scale factor when performing text autosizing of
 # websites that do not set an explicit viewport description.
 deviceScaleAdjustment type=double, initial=1.0, invalidate=TextAutosizing