Add default implementations of metrics and field trial to 'webrtc' target.

Having this, the produced libwebrtc.a library becomes complete and is
more useful to most downstream users. More advanced use cases that needs to
wire up their own field trial and metrics implementations will need to
create their own target.

BUG=webrtc:7372
NOTRY=True
CC=agouaillard@gmail.com

Review-Url: https://codereview.webrtc.org/2840553002
Cr-Commit-Position: refs/heads/master@{#17860}

webrtc/BUILD.gn

@@ -257,7 +257,7 @@ if (!build_with_chromium) {
       "pc",
       "sdk",
       "stats",
-      "system_wrappers",
+      "system_wrappers:system_wrappers_default",
       "video",
       "voice_engine",
     ]