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This adds a buildflag that enables handling encrypted content in the HW
accelerated video decoders for Chrome OS. Initially this will be for
VAAPI only. No builders are enabling this flag yet, it is still in
development.

BUG=b:153111783
TEST=Built/deployed Chrome with flag enabled/disabled

Change-Id: Idcf50f3c302d619f8586306db9cf85164e051dee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461993
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815424}