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This updates libsrtp to 1.5.2 and uses the OpenSSL/BoringSSL crypto code.
See https://codereview.chromium.org/889083003/ and https://codereview.chromium.org/1098043003/

Summary of changes available at:
https://chromium.googlesource.com/chromium/deps/libsrtp/+log/6446144..9c53f85

BUG=328475

Review URL: https://codereview.chromium.org/981593002

Cr-Commit-Position: refs/heads/master@{#328972}