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 This change makes the textfield uneditable. The text can
still be updated via API (which is necessary to show URL
in popup).

BUG=none
TEST=uadded new ReadOnlyTest

Review URL: http://codereview.chromium.org/6314012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72022 0039d316-1c4b-4281-b951-d872f2087c98