Fix radio items in MenuGtk.
Fix radio items in MenuGtk.

Suppose there are three radio items A, B, C, and A is now being
checked. If you click C, commands associated with A and C are both
executed. This is because A also receives the "activate" signal,
before C receives it.

BUG=30450
TEST=try; manually with gdb with the steps described in crbug.com/30450.

Review URL: http://codereview.chromium.org/506010

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@34647 0039d316-1c4b-4281-b951-d872f2087c98

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