PrintTo functions for RTCStats added in rtcstatscollector_unittest.cc
PrintTo functions for RTCStats added in rtcstatscollector_unittest.cc

Future test code will do stuff like EXPECT_EQ(report, expected_report).
They're all defined in the unittest because it and stats' operator==
is only used for testing.

See https://cs.chromium.org/chromium/src/testing/gtest/include/gtest/gtest-printers.h?sq=package:chromium&dr=C&rcl=1477394469&l=707

BUG=chromium:627816

Review-Url: https://codereview.webrtc.org/2445343003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14779}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc