Touch flings should not bubble.

Currently touch flings can bubble out of an accelerated overflow scrolling div,
but this should not happen.

Looking at InputHandlerProxy::HandleGestureFling, FlingScrollBegin appears to be used for touch fling. Given that this is the case, I've set should_bubble_scrolls_ to false there.

Tests:
TouchFlingShouldNotBubble
 - checks that this fling doesn't bubble.
WheelFlingShouldBubble
 - checks that this does.

R=danakj@chromium.org,aelias@chromium.org
BUG=270281

Review URL: https://codereview.chromium.org/26470004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227585 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227585