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Currently the CDM process is a per-CDM-type singleton shared by all
user profiles and all sites. This CL makes it per-CDM-type,
per-user-profile and per-site for security and privacy improvements.

See go/cdm-process-site-isolation for details.

This CL also adds a new base::Feature media::kCdmProcessSiteIsolation to
control this feature. It is enabled by default.

Manually tested by running 3 sites in one user profile and 1 site in
guest mode, with all sites serving protected content using Widevine key
system. With this CL, a maximum of 4 CDM processes are created. Without
this CL or when media::kCdmProcessSiteIsolation is disabled, there is
always at most one CDM process.

Bug: 638950, 1077390
Test: See above
Change-Id: I326795648111cc7f9436a4389076d92be437d6d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204686Reviewed-by: Aaron Colwell <acolwell@chromium.org>
Reviewed-by: John Rummell <jrummell@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770607}