Made context menu aware of pinch viewport offset

Fixed issue where context menu wouldn't take into account the pinch
viewport's offset so the menu would appear in the wrong place if the user
was pinched in.

BUG=371441

Review URL: https://codereview.chromium.org/301433003

git-svn-id: svn://svn.chromium.org/blink/trunk@174820 bbb929c8-8fbe-4397-9dbb-9b2b20218538