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If using shared memory, stores base::Features in the field trial
allocator (under a different type defined in FeatureList) when
spawning the first new process. When a child process is created,
it will first try to get initialized from shared memory. If that
doesn't work, it'll fallback to the command line.

BUG=661224

Review-Url: https://codereview.chromium.org/2546653002
Cr-Commit-Position: refs/heads/master@{#436311}