Truncating X event's coordinate instead of rounding

In CL https://codereview.chromium.org/69653002
I was thinking using rounding instead of truncating when converting
X event's coordinate to ui::TouchEvent's location seems more correct.
But it introduces the bug that an edge event can be rounded to location
that is out of bounds, e.g. rightmost X event's x position on Pixel is
2559, and it gets divided by device scale factor of 2 so becomes
1279.5 and rounded to be 1280, which is out of bounds.
So lets revert to the truncating behavior.

BUG=328151
TEST=follow the test at #1 & #5 at crbug.com/328151 and make sure
     the issue is fixed.

Review URL: https://codereview.chromium.org/118423003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241746 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241746