RTCStats equality operator added.

This will be helpful in unittests to EXPECT_EQ reports. It should be a
useful operator to have outside of testing as well.

BUG=chromium:627816
NOTRY=True

Review-Url: https://codereview.webrtc.org/2441543002
Cr-Original-Commit-Position: refs/heads/master@{#14767}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 67c8bc4bf2cc10669e5cb7fd461469f30edcaa23