GTK: fix sizing of js alert dialogs.
GTK: fix sizing of js alert dialogs.

Determine the size request after showing the dialog for the first time. For one reason or another, the size request before that point doesn't take into account the checkbox label text.

BUG=32394
TEST=see bug

Review URL: http://codereview.chromium.org/549073

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@36438 0039d316-1c4b-4281-b951-d872f2087c98

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