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Added thread_checker in StreamProcessorHelper, SysmemBufferPool and
SysmemBufferWriterQueue. Also added sequence_checker in
Fuchsia*StreamDecryptor.

Change-Id: I467e53f3512bac5f105d18f6fa5d67fc01fa3d6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1838625
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704339}