Cancel drag if the bookmark model changes.
Cancel drag if the bookmark model changes.

When we rebuild the bookmark bar (e.g. something changed), clear the
dragging pasteboard to avoid stale pointers.

BUG=34522
TEST=see bug; it's quite clear.

Review URL: http://codereview.chromium.org/836004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@41392 0039d316-1c4b-4281-b951-d872f2087c98

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