external/boringssl: Sync to 915c121bb5d424e09bf05c3aabf172a44e958e28.

This includes the following changes:

https://boringssl.googlesource.com/boringssl/+log/ea52ec98a56a40879b37493f3d1da1a1679e1fba..915c121bb5d424e09bf05c3aabf172a44e958e28

Test: BoringSSL CTS Presubmits
Change-Id: I3f5eba69372b484e19f4ca250c81f208aa5d3dc5
55 files changed