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Currently there are const char* and std::string varieties of strings in
RTCStatsMember objects ("StaticString" and "String"). The const char*
version is going away so that we don't have two string types.

This CL removes references to the "StaticString" types so that a WebRTC
CL can remove it without breaking Chromium. Only rtc_stats.cc have to
be updated, not affecting any code because rtc_stats is not used yet
(a CL is in the works that will use it and add unittests for it that is
blocked on a WebRTC CL that is under review).

BUG=chromium:627816

Review-Url: https://codereview.chromium.org/2340603004
Cr-Commit-Position: refs/heads/master@{#418845}