[Chromecast] Don't include field_trial in overrides.

Cast builds always use the webrtc/system_wrappers, but the
webrtc_overrides get pulled in aswell via transient dependencies, which
ends up causing a name collision that lld cannot resolve. (In particular,
field_trial FindFullName was colliding.)

Bug: None
Change-Id: I5c2fc9119c9806f344ee0f599e188ba98e2e5f15
Reviewed-on: https://chromium-review.googlesource.com/744424
Commit-Queue: Mike Bjorge <mbjorge@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513291}

third_party/webrtc_overrides/BUILD.gn

@@ -2,6 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/chromecast_build.gni")
+
 group("webrtc_overrides") {
   public_deps = [
     ":webrtc",
@@ -59,11 +61,14 @@ static_library("webrtc") {
   }
 
   if (is_nacl) {
-    # For NACL, we have to add a default implementation for field_trail.
+    # For NACL, we have to add a default implementation for field_trial.
     deps += [
       "//native_client_sdk/src/libraries/nacl_io",
       "//third_party/webrtc/system_wrappers:field_trial_default",
     ]
+  } else if (is_chromecast) {
+    # Cast builds use the default implementation for field_trial.
+    deps += [ "//third_party/webrtc/system_wrappers:field_trial_default" ]
   } else {
     # Otherwise, we just add the field_trial which redirects to base.
     sources = [