Add a setting to disable threaded scrolling

Source/core/frame/Settings.in

Index: Source/core/frame/Settings.in
diff --git a/Source/core/frame/Settings.in b/Source/core/frame/Settings.in
index c629a034cc0ac2d5bd881cf371441054b056452a..02b83c8b46e96c8fe4f35fbbd7e408666e84f3a3 100644
--- a/Source/core/frame/Settings.in
+++ b/Source/core/frame/Settings.in
@@ -69,6 +69,7 @@ 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