Fix extensions NSS/BoringSSL dependencies
The NSS dependency should be guarded by a use_openssl check. BoringSSL and NSS
cannot be linked together as they define some of the same symbols. Update BUILD.gn
analogously by depending on //crypto:platform.
Also condition nss in build/all.gyp on use_openssl in preparation for
https://codereview.chromium.org/
694643002/ which will disable the nss and nspr
targets altogether.
BUG=429021
Review URL: https://codereview.chromium.org/
686693004
Cr-Commit-Position: refs/heads/master@{#302325}