WebUI: Fix issue with context menu button stealing focus from list

This regressed in r25605004

When the user mousedown on the context menu button we used to call
preventDefault on the event. With r25605004 we unconditionally call
focus on the button.

Now we do not call focus on elements if preventDefault was called.

BUG=307386
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/26997003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228914 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228914