Fixed touch selection handle manipulation when pinched in.

This patch applies the pinch viewport offset to selection handle
coordinates when passing the coordinates into Blink. This fixes the case
where selection handles would appear in the wrong place while being
manipulated while pinched in or when the ChromeOS on-screen keyboard is
showing.

BUG=376332

Review URL: https://codereview.chromium.org/319643002

git-svn-id: svn://svn.chromium.org/blink/trunk@175682 bbb929c8-8fbe-4397-9dbb-9b2b20218538