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Add SANDBOX_TYPE_WIDEVINE, and a policy for it. This value is
also special in that will also defers some initializations in a
subsequent CL.

Also parse --sandbox-type=ppapi for utility processes as
SANDBOX_TYPE_PPAPI, as a generic equivalent.

Tidy GetGpuProcessSandbox() since there was a blown merge around it
and I was looking at it.

TBR=jam@chromiuim.org

Bug: 765840
Change-Id: Ifdc0949f811c916b12f6d89b4fbd1ffdab61cd64
Reviewed-on: https://chromium-review.googlesource.com/671453
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502956}