gtk: Gray out uneditable search URLs.
gtk: Gray out uneditable search URLs.

Change the appearence of the url entry to be like disable, so the user
doesn't begin confused.

BUG=34476
TEST=see bug

Review URL: http://codereview.chromium.org/2823030

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@51093 0039d316-1c4b-4281-b951-d872f2087c98

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