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In normal cases the session ID returned by the MediaFoundation CDM
(IMFContentDecryptionModule) should never be the same. In the rare case
duplicate session IDs are returned, the current implementation will
crash.

This CL fixes the implementation and adds a unit test.

NOPRESUBMIT=true

Bug: 999747
Change-Id: I319c33517dd6452cb45ee30d3c1c02a41b0c6ccf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278416Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785817}