Update WebRTC's C++ style guide to reflect the switch to C++14.
Update WebRTC's C++ style guide to reflect the switch to C++14.

No-Try: True
Bug: webrtc:10945
Change-Id: Ife5d5c12144e00aeefd5ccfe8470c8741ad8eb54
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151460
Commit-Queue: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Reviewed-by: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Reviewed-by: Danil Chapovalov <55de8d9c2b5977da2dd48f967f21f9d606b172a5@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29194}

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