Expose setting API to set ImageAnimationPolicy(Allowed, Once, None)

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 230 matching lines @@
 accessibilityFontScaleFactor type=double, initial=1.0, invalidate=TextAutosizing
 
 
 # Only set by Layout Tests.
 mediaTypeOverride type=String, initial="screen", invalidate=MediaType
 
 # loadsImagesAutomatically only suppresses the network load of
 # the image URL. A cached image will still be rendered if requested.
 loadsImagesAutomatically initial=false, invalidate=ImageLoading
 imagesEnabled initial=true, invalidate=ImageLoading
+imageAnimationPolicy type=ImageAnimationPolicy, initial=ImageAnimationPolicyAllowed
 
 javaEnabled initial=false
 pluginsEnabled initial=false
 
 viewportEnabled initial=false, invalidate=ViewportDescription
 viewportMetaEnabled initial=false
 
 dnsPrefetchingEnabled initial=false, invalidate=DNSPrefetching
 
 # FIXME: This is a temporary flag and should be removed
@@ skipping 42 matching lines @@
 primaryHoverType type=HoverType, initial=HoverTypeNone, invalidate=MediaQuery
 
 # Whether accessibility support is enabled at all.
 accessibilityEnabled initial=false, invalidate=AccessibilityState
 
 # If true, the value in password fields is exposed to assistive technologies.
 accessibilityPasswordValuesEnabled initial=false
 
 # If true, static text nodes expose inline text box children.
 inlineTextBoxAccessibilityEnabled initial=false