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This CL changes the iOS port to use IOSChromeMetricsServiceClient rather than
its forked version of ChromeMetricsServiceClient. There is one intended
functional change in that
ChromeMetricsServiceClient::RecordCommandLineMetrics() was not ported to
IOSChromeMetricsServiceClient; see crbug.com/375794 for details.  Other than
that, there should be no functional changes.

BUG=512421, 375794

Review URL: https://codereview.chromium.org/1470963002

Cr-Commit-Position: refs/heads/master@{#361103}