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Re-land of https://codereview.chromium.org/2534633002/

In order to have a perf test looking at stats of interest we need to be
able to get the results of a JavaScript getStats call. These can be
returned as JSON-stringified JavaScript dictionaries.

In this CL:
- [g|G]etStatsReportDictionary added to webrtc_browsertest_base.cc and
  peerconnection_getstats.js, which produce/read a JSON-stringified
  version of the stats.
- RTCStatsReportDictionary and RTCStatsDictionary, helper classes for
  reading stats from the base::Dictionary representation of the stats.
- Unittests for the helper classes.

In a follow-up CL these methods/classes will be used to obtain the
stats in the to-be-added performance test. This will yield pretty
graphs.

BUG=670306, 627816

Review-Url: https://codereview.chromium.org/2543173002
Cr-Commit-Position: refs/heads/master@{#436252}