Add supports_native_initiated_connections to native messaging host manifests.

If a native messaging host sets supports_native_initiated_connections to
true, it opts into messaging connections initiated by a native app and
receiving additional command line switches instructing it the invocation
to perform such connection initiation. These will be implemented in
follow-up CLs.

Add a base::Feature for onConnectNative and gate
supports_native_initiated_connections parsing behind it.

Bug: 967262
Change-Id: Ia8e4280b157984cabe8779c8437eaf643fa5f950
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1630072
Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668267}