Widget::Show() should not active widgets that can't be activated.

Basically, if Widget::CanActivate() returns false, the widget should not
be activated.

BUG=336163
COLLABORATOR=benwells@chromium.org

Review URL: https://codereview.chromium.org/143743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250242 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250242