We're now ready https://codereview.webrtc.org/1984503002/ downstream,
so make sure we can enable libevent but still choose which libevent
implementation to use. This follows the common pattern where an enable_
flag controls whether we should use the feature at all, whereas build_
controls if we should use the dependency from our DEPS file or
something else.
NOTRY=True
Review-Url: https://codereview.webrtc.org/1980003002
Cr-Commit-Position: refs/heads/master@{#12772}
ff274394fe