Removes enable_webrtc from build/config/features.gni and puts it in
media/media_options.gni.

Converts the ENABLE_WEBRTC from a define set globally to being set in the
media_features.h generated buildflag header.

R=sky@chromium.org
TBR=nasko@chromium.org (*_messages.h)
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/2517953003 .

Cr-Commit-Position: refs/heads/master@{#433912}