Style guide: Add text about C and non-compliant C++ code
Style guide: Add text about C and non-compliant C++ code

BUG=none
NOTRY=true

Change-Id: I64a64a4f138b6c8aa24ad2266284024d50163908
Reviewed-on: https://chromium-review.googlesource.com/648971
Reviewed-by: Niels Möller <0057859e47d61d90529d0f66d9ef692cefedb46a@chromium.org>
Commit-Queue: Karl Wiberg <2c708eb0a58047bf403399f50b3da33ffd2dd444@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#19680}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc