Merge third_party/opus/src from https://chromium.googlesource.com/chromium/deps/opus.git at 6b6bee25314cfac02cc555cddedb9680c63a26d6

This commit was generated by merge_from_chromium.py.

Change-Id: I21d8ae69c1a3c4d1df13015f82099a7d6422678e