Remove deprecated build targets from webrtc_overrides.

The build target //webrtc/system_wrappers:field_trial_default is
deprecated. WebRTC will always link the default field_trial
implementation unless rtc_exclude_field_trial_default is true.

The default value of rtc_exclude_field_trial_default keeps Chromium
needs into account (see [1]).

[1] - https://cs.chromium.org/chromium/src/third_party/webrtc/webrtc.gni?l=44-51&rcl=95c56eebe0a2b31ad5752138d15b431124e17d36

Bug: webrtc:9631
Change-Id: Ie603530c8bb3b10b3fcbee47398dc69085e5f1e8
Reviewed-on: https://chromium-review.googlesource.com/1226143
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594656}

third_party/webrtc_overrides/BUILD.gn

@@ -57,19 +57,17 @@ static_library("webrtc") {
     cflags = [ "/wd4005" ]
   }
 
+  # When WebRTC is built as part of Chromium it should exclude the default
+  # implementation of field_trial unless it is building for NACL or Chromecast.
+  # This configuration happens here:
+  # https://cs.chromium.org/chromium/src/third_party/webrtc/webrtc.gni?l=44-51&rcl=95c56eebe0a2b31ad5752138d15b431124e17d36
   if (is_nacl) {
-    # For NACL, we have to add a default implementation for field_trial and
-    # runtime_enabled_features.
     deps += [
       "//native_client_sdk/src/libraries/nacl_io",
-      "//third_party/webrtc/system_wrappers:field_trial_default",
       "//third_party/webrtc/system_wrappers:runtime_enabled_features_default",
     ]
   } else if (is_chromecast) {
-    # Cast builds use the default implementation for field_trial and
-    # runtime_enabled_features.
     deps += [
-      "//third_party/webrtc/system_wrappers:field_trial_default",
       "//third_party/webrtc/system_wrappers:runtime_enabled_features_default",
     ]
   } else {