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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

This previously landed in https://codereview.chromium.org/1130413005/ but got
reverted in https://codereview.chromium.org/1139233002/ as it broke the Chrome
Linux x64 builds due to a added dependency to "libgcc_s.so.1(GCC_3.0)(64bit)".
The dependency was already allowed for 32bit, so it is allowed for 64bit now,
too.

BUG=328475

Review URL: https://codereview.chromium.org/1144623002

Cr-Commit-Position: refs/heads/master@{#330209}