Disables showing tooltips while a system menu is showing on windows

When a system menu is showing we continue to get and receive mouse
events while the mouse is over the menu. The tooltip code thinks the
mouse is over the aura::Window and will then show the tooltip. The fix
is to disable tooltips while system menus are showing.

I'm also making the only way to enable/disable tooltips be by way of a
scoper. This should hopefully be less error prone.

BUG=327970
TEST=none
R=varunjain@chromium.org

Review URL: https://codereview.chromium.org/109403008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243350 0039d316-1c4b-4281-b951-d872f2087c98
Nukes Link::OnPaint, it's not needed and causes double focus border

Label::OnPaint also draws a focus border, so Link doesn't need to do
it.

BUG=330675
TEST=none
R=ben@chromium.org

Review URL: https://codereview.chromium.org/124553006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243351 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243350
SVN changes up to revision 243351