GN: Put BoringSSL Chromium specific targets inside a condition

The boringssl_unittests target depends on Chromium's base,
so having it being processed by default results in errors
for other projects reusing the GN files for BoringSSL that
are located in the Chromium repo.

TBR=kjellander@chromium.org, davidben@chromium.org
BUG=webrtc:5829, 606944

Review URL: https://codereview.chromium.org/1923683002

Cr-Commit-Position: refs/heads/master@{#389932}
3 files changed