Merge third_party/libsrtp from https://chromium.googlesource.com/chromium/deps/libsrtp.git at 2f2e557b809dc1dd7977098ad354a540f04beb4e

This commit was generated by merge_from_chromium.py.

Change-Id: Ie0c81043b4026621642da2f3bfe1650dfe341faa