Merge third_party/libsrtp from https://chromium.googlesource.com/chromium/deps/libsrtp.git at 662d81de5cbf3667f1baaafe051b8b4ab0b12fe2

This commit was generated by merge_from_chromium.py.

Change-Id: I228102856110fba75530b2ffa9e463acb75967d2