touchui: Activate a visible top-level Widget if there's no active Widget.
touchui: Activate a visible top-level Widget if there's no active Widget.

This also lets touch-events go to the activated widget if the activation
happened because of the touch.

BUG=101727
TEST=manually


Review URL: http://codereview.chromium.org/8481004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@108803 0039d316-1c4b-4281-b951-d872f2087c98

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