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This CL removes the old renderer appcontainer flag from about:flags.
It also removes the old command line switch to enable or disable, instead
replacing it with a feature. This feature is disabled by default but the
expectation is to enable by default in the next milestone.

Bug: 501975
Change-Id: If9b7425624d60243e998958d31e53d5607a3424e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761471Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: James Forshaw <forshaw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688552}