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Currently seeing a crash when two iframes from the same origin request the
same permission. The issue arises because they have the same ID and therefore
the same entry in the hash map, which makes the request_ptr destruct. This CL
cancels the second (redundant) permission request.

BUG=433877

Review URL: https://codereview.chromium.org/799783002

Cr-Commit-Position: refs/heads/master@{#313673}