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This commit makes the technical changes required to make the wake lock code
in Blink aware of workers (i.e. use ExecutionContexts instead of Documents,
exposing the WakeLock interface to dedicated workers and making
WakeLockController support both Documents and DedicatedWorkerGlobalScopes).

From a user perspective, though, this is still not very useful, as dedicated
workers cannot acquire screen locks, and right now we always deny all
requests for system locks.

Bug: 967667
Change-Id: I6a775b1aac2b600006cb6f2ce0595514548bff17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702378Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#678297}