Always disconnect event listeners when clearing for navigation, even if the script context is empty.
Always disconnect event listeners when clearing for navigation, even if the script context is empty.

R=mbelshe
BUG=13266
TEST=gmail shouldn't crash anymore.

Review URL: http://codereview.chromium.org/119109

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@17546 0039d316-1c4b-4281-b951-d872f2087c98

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