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This allows the use of e.g.:

WeakHandle<T> weak_handle = MakeWeakHandle(WeakPtr<U>());

as long as U is convertible to T, instead of:

WeakHandle<T> weak_handle = WeakHandle<T>(WeakPtr<U>());

Fix all callers.

BUG=
TEST=


Review URL: http://codereview.chromium.org/8568041

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110436 0039d316-1c4b-4281-b951-d872f2087c98