webui: Hide a context menu when the parent window loses focus

BUG=24820
TEST=manually tested
R=nkostylev@chromium.org

Review URL: https://codereview.chromium.org/27522003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229336 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229336