Prepare Blink for removal of unsafePluginPastingEnabled 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 68 matching lines @@
 usesEncodingDetector initial=false
 allowScriptsToCloseWindows initial=false
 regionBasedColumnsEnabled initial=false
 
 # FIXME: This should really be disabled by default as it makes platforms that
 # 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
+unsafePluginPastingEnabled initial=false
 treatIPAddressAsDomain initial=false
 
 preferCompositingToLCDTextEnabled initial=false, invalidate=AcceleratedCompositing
 
 # 3D canvas (WebGL) support.
 webGLEnabled initial=false
 
 webGLErrorsToConsoleEnabled initial=true
 accelerated2dCanvasEnabled initial=false
 antialiased2dCanvasEnabled initial=true
@@ skipping 221 matching lines @@
 showContextMenuOnMouseUp initial=false
 
 disableReadingFromCanvas initial=false
 strictMixedContentChecking initial=false
 strictPowerfulFeatureRestrictions initial=false
 logDnsPrefetchAndPreconnect initial=false
 
 # If true, the UA styles will include viewportAndroid.css. It is used for
 # Android, and dynamically set by the inspector for mobile emulation.
 useMobileViewportStyle initial=false, invalidate=ViewportRule