Correct the position of dragstart event

This patch is to fix an issue that the x and y position of dragstart event
is not correct if it is triggered by long press event.

BUG=None
TEST=fast/events/touch/gesture/long-press-on-draggable-element-triggers-drag.html

Review URL: https://chromiumcodereview.appspot.com/14598021

git-svn-id: svn://svn.chromium.org/blink/trunk@150809 bbb929c8-8fbe-4397-9dbb-9b2b20218538
2 files changed