The warning previously suppressed made it possible to define tings like
constructors in the header, and "complex" objects did not need to have
an explicit out-of-line copy constructor, destructor, etc.
To be able to not suppress this warning, the RTCStats macro was split
into a WEBRTC_RTCSTATS_DECL() and WEBRTC_RTCSTATS_IMPL() for .h and .cc
respectively. Some copy constructors are also defined.
BUG=chromium:627816
Review-Url: https://codereview.webrtc.org/2373503002
Cr-Commit-Position: refs/heads/master@{#14545}
fc5e0504ea