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This is a reland of 5c4908e8

Original change's description:
> macOS V2 Sandbox: Remove the feature flags.
>
> The V2 sandbox is now default for all but the network and GPU processes
> on macOS. This CL deletes the feature flag. A new experiment will be
> introduced for the GPU process. The default case for the sandbox profile
> switch is removed, because previous CHECK() failures were confusing to
> diagnose. All profiles must now be handled explicitly, so Chrome won't
> build if a new profile type isn't handled.
>
> Bug: 749839
> CQ-Depend: 1268671
> Change-Id: I0860d3a5148b0085be6a99dc01c4971602ee5582
> Reviewed-on: https://chromium-review.googlesource.com/c/1269996
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Reviewed-by: Mark Pearson <mpearson@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Commit-Queue: Greg Kerr <kerrnel@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#598128}

CQ-DEPEND=CL:1268671

Bug: 749839
Change-Id: I91544907b640c572823f3aae3e29c0cfa003da51
Reviewed-on: https://chromium-review.googlesource.com/c/1274888Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Greg Kerr <kerrnel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598822}