Adjust aura synthetic mouse event locations to screen location.

This is an attempt to fix flaky page scroll unittests in win aura.

BUG=328970
BUG=323249
BUG=297960

Review URL: https://codereview.chromium.org/117333006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242062 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242062