Allow raw pixel scrolling in web contents in Aura.

A future patch will allow raw pixel scrolling in views.

ui::LocatedEvent stores a floating point location. Currently
integral data is returned for all previously existing
accessors. In the future, all accessors should return the
floating point values.

BUG=329354
TEST=GestureRecognizerTest.GestureEventScroll

Review URL: https://codereview.chromium.org/143603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247358 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247358