linux: explicitly unhighlight buttons before disabling
linux: explicitly unhighlight buttons before disabling

If your mouse is hovering the back button when the button is disabled
(i.e. you click back and it takes some time for the navigate to go
through), when the button is re-enabled GTK restores the highlighted
state to the button.

I reproduced this in a very simplified naive app on the bug and had
the same result, so I think this workaround is ok.

BUG=13729
TEST=do the actions listed in the bug

Review URL: http://codereview.chromium.org/1521016

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@43779 0039d316-1c4b-4281-b951-d872f2087c98

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