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This CL adds MediaEngineNotifyImpl to implement IMFMediaEngineNotify
required by IMFMediaEngine (https://docs.microsoft.com/en-us/windows/win32/api/mfmediaengine/nn-mfmediaengine-imfmediaengine).

Explainer:
https://docs.microsoft.com/en-us/windows/win32/medfound/protected-media-path

Design Doc:
https://docs.google.com/document/d/1kXMdmfsSKU3tcwTxG-wFU-MaEA6sN3by45PCGBV8zJU

Bug: 999747
Change-Id: I082f3dd29dcfc8fb409d387af040aa1c97ff79aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064454Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Frank Li <frankli@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#743858}