GTK: Try to position info bubbles onscreen.

I didn't go so far as to add support for arrows-on-the-bottom
in this change; I'm just making the bubble extend left or right
as needed.  I'll add a parameter to specify the default arrow
location (needed for browser action popups) in another CL.

Tested by:
- dragging a window to the right edge of the screen and
  confirming that bookmark bubbles open extended to the left
- opening a bookmark bubble, using a little X program to move
  the Chrome window to the right side of the screen (can't drag
  it there since the pointer is grabbed), and confirming that
  the bubble gets updated to extend to the left
- running in Arabic and confirming that bubbles extend to the
  left by default but get switched to extend to the right as
  needed

BUG=23373
TEST=see above

Review URL: http://codereview.chromium.org/316006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@29992 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed