Remove the forceCompositingMode 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 81 matching lines @@
 xssAuditorEnabled initial=false
 unsafePluginPastingEnabled initial=true
 treatIPAddressAsDomain initial=false
 
 acceleratedCompositingForVideoEnabled initial=true, invalidate=AcceleratedCompositing
 acceleratedCompositingForCanvasEnabled initial=true, invalidate=AcceleratedCompositing
 acceleratedCompositingForFiltersEnabled initial=false, invalidate=AcceleratedCompositing
 acceleratedCompositingForFixedPositionEnabled initial=false, invalidate=AcceleratedCompositing
 acceleratedCompositingForOverflowScrollEnabled initial=false, invalidate=AcceleratedCompositing
 acceleratedCompositingForFixedRootBackgroundEnabled initial=false
-
-forceCompositingMode initial=true, invalidate=AcceleratedCompositing
-
-# Works only in conjunction with forceCompositingMode.
 compositedScrollingForFramesEnabled initial=false, invalidate=AcceleratedCompositing
 
 # 3D canvas (WebGL) support.
 webGLEnabled initial=false
 
 webGLErrorsToConsoleEnabled initial=true
 accelerated2dCanvasEnabled initial=false
 antialiased2dCanvasEnabled initial=true
 accelerated2dCanvasMSAASampleCount type=int, initial=0
 
@@ skipping 155 matching lines @@
 
 # This value indicates the maximum number of bytes a document is allowed
 # to transmit in Beacons (via navigator.sendBeacon()) -- Beacons are
 # intended to be smaller payloads transmitted as a page is unloading, not
 # a general (one-way) network transmission API.
 # The spec ( https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/Beacon/Overview.html )
 # does not proscribe an upper limit, but allows for it -- the underlying
 # API will return 'false' in that case.
 maxBeaconTransmission type=int, initial=16384