The chromium-stable-with-bazel branch is no longer being updated. Historically, this branch pointed to, though lagged behind, the revision of BoringSSL that the current latest stable release of Chromium used. It was used by projects that wanted some slower-moving branch to follow than HEAD.
BoringSSL now tags periodic releases that can be used instead. See the tags of the form 0.YYYYMMDD.N. These releases contain Bazel builds.