Properly handle scroll and fling gestures after pinch zoom

Introduce bookkeeping to clear the scrolling layer if none existed prior to
PinchBegin.  The actively scrolling layer will be cleared on PinchEnd, if
necessary.  This fixes an otherwise invalid assumption made by InputHandlerProxy
that a scroll sequence after an unhandled ScrollBegin gesture will not result in
an actively scrolling layer. 

BUG=287776

Review URL: https://chromiumcodereview.appspot.com/23859014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224875 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224875