Anchor the high contrast bubble to the app menu.

Anchor the bubble to Chrome's app menu; fix the arrow.
Only try to show the bubble if the app menu exists.

BUG=350508
R=sky@chromium.org
TEST=When using the high-contrast-black theme, the chrome bubble shows anchored reasonable.

Review URL: https://codereview.chromium.org/189513007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255855 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255855