linux_aura: Put toolbar dropdown menus on the screen with the cursor.

When right clicking on, for example, the back button, the dropdown menu
should be positioned on the screen with the cursor, so that this picks
the right screen even when the back button straddles two screens,
instead of always picking the left screen.

BUG=339536
R=msw@chromium.org

Review URL: https://codereview.chromium.org/157843003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250190 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250190