Add a setting to disable threaded scrolling

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 51 matching lines @@
 allowUniversalAccessFromFileURLs initial=true
 allowFileAccessFromFileURLs initial=true
 javaScriptCanOpenWindowsAutomatically initial=false
 supportsMultipleWindows initial=true
 javaScriptCanAccessClipboard initial=false
 shouldPrintBackgrounds initial=false
 shouldClearDocumentBackground initial=true
 
 textAreasAreResizable initial=false, invalidate=Style
 acceleratedCompositingEnabled initial=true, invalidate=AcceleratedCompositing
+acceleratedScrollingEnabled initial=true, invalidate=Style

[Rick Byers, 2014/09/15 21:00:05]

I've been a little unclear on what exactly we use the term "accelerated" to
mean.  I think it sometimes means "uses the compositor to do transforms rather
than repainting" and other times means "runs on the impl thread".  If it's not
too much hassle, I'd prefer a less ambiguous term here - eg.
"threadedScrollingEnabled".  If my understanding is wrong and our use of
"accelerated" is unambiguous, then this is fine as is. 

Also please a comment describing when this setting should be used, ideally
linking to a bug that will track eventually removing it (maybe just
crbug.com/410974). 
See
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/984HMZyFQNo/AV...
for discussion.

[jdduke (slow), 2014/09/15 21:02:07]

Yeah, I use "threaded" in the Chromium side, I was just patterning after this
verbiage (they also use threadedCompositingEnabled for this setting in
Chromium).

 
 shrinksStandaloneImagesToFit initial=true
 
 # FIXME: Does this do anything now that we don't support page cache?
 pageCacheSupportsPlugins initial=false
 
 offlineWebApplicationCacheEnabled initial=false
 usesEncodingDetector initial=false
 allowScriptsToCloseWindows initial=false
 deferredFiltersEnabled initial=true
@@ skipping 206 matching lines @@
 # has pointerType coarse *and* fine).
 availablePointerTypes type=int, initial=PointerTypeNone, invalidate=MediaQuery
 availableHoverTypes type=int, initial=HoverTypeNone, invalidate=MediaQuery
 
 # These values specify properties of the user's primary pointing device only.
 primaryPointerType type=PointerType, initial=PointerTypeNone, invalidate=MediaQuery
 primaryHoverType type=HoverType, initial=HoverTypeNone, invalidate=MediaQuery
 
 accessibilityEnabled initial=false
 inlineTextBoxAccessibilityEnabled initial=false