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1. Turn on flag 'enable_widevine' and
'ENABLE_PROTECTED_MEDIA_IDENTIFIER_PERMISSION'.

2. Add widevine key system config.

Bug: 991723
Test: Shaka player demo "Dig the Uke".
Change-Id: I49a7661fb4b8b5732b597622f9d496ea611dc9ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783802
Commit-Queue: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694945}