Adds a SetWindowPos(HIDE) to tooltip to avoid flicker

For some reason without this the tooltip occasionally flickers at 0x0
before snapping to original location. No idea why it's necessary.

BUG=307861
TEST=see bug
R=ananta@chromium.org

Review URL: https://codereview.chromium.org/26852006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229558 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229558