Introduce an empty WebRTC component.

While testing the WebRTC component build in [1], there was a problem
with some testonly code Chromium depends on (which cannot be part of the
WebRTC component).

In order to support this use case, [2] will make the testonly code
needed by Chromium depend on the WebRTC component and this CL is needed
in order to land everything in an incremental fashion.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1868961
[2] - https://webrtc-review.googlesource.com/c/src/+/157427

Bug: webrtc:9419
Change-Id: Ie995af67a7ab4ba1a2e5bb012f2e9b33be056acb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872937Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707985}

third_party/webrtc_overrides/BUILD.gn

@@ -10,6 +10,10 @@ group("webrtc_overrides") {
   ]
 }
 
+component("webrtc_component") {
+  public_deps = []
+}
+
 source_set("webrtc") {
   public_deps = [
     # TODO(kjellander): Start cleaning up this target as soon as