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This implements support for Rappor metrics which contain
multiple fields that support correlation analysis.
Example:
scoped_ptr<Sample> sample = rappor_service->CreateSample(COARSE_RAPPOR_TYPE);
sample->SetStringField("Domain", "google.com");
sample->SetFlagsField("Flags", 0x10, 8 /* # of bits */);
rappor_service->RecordSampleObj("MyMetric", sample.Pass());

This change also removes metric_name from the personalization string for the PRR.  This means that two different metrics which report the same value will use the same PRR, which will improve privacy when multiple metrics report the same value.

BUG=451647

Review URL: https://codereview.chromium.org/1090683003

Cr-Commit-Position: refs/heads/master@{#327207}