weblayer: fix X-Client-Data header sending

There were two issues:
. Need to register the VariationsIdsProvider with the trial-registry.
  Without this the header content is never updated.
. Need to return true from VariationsClient::IsSignedIn(). This is
  needed as registerExternalExperimentIDs() supplies SignedIn.

I clearly need a test for this. Will add it in a follow up.

BUG=11478276
TEST=none

Change-Id: I62b6976dd51c3eb3aff4412ee2512b7c5516301b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532516Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826834}

weblayer/browser/android/metrics/weblayer_metrics_service_client.cc

@@ -15,6 +15,7 @@
 #include "components/metrics/metrics_provider.h"
 #include "components/metrics/metrics_service.h"
 #include "components/page_load_metrics/browser/metrics_web_contents_observer.h"
+#include "components/variations/variations_ids_provider.h"
 #include "components/version_info/android/channel_getter.h"
 #include "content/public/browser/browser_context.h"
 #include "google_apis/google_api_keys.h"
@@ -140,6 +141,8 @@ void WebLayerMetricsServiceClient::OnMetricsStart() {
     std::move(task).Run();
   }
   post_start_tasks_.clear();
+  GetMetricsService()->synthetic_trial_registry()->AddSyntheticTrialObserver(
+      variations::VariationsIdsProvider::GetInstance());
 }
 
 void WebLayerMetricsServiceClient::OnMetricsNotStarted() {

weblayer/browser/browser_context_impl.cc

@@ -306,10 +306,13 @@ class BrowserContextImpl::WebLayerVariationsClient
   }
 
  private:
-  bool IsSignedIn() const {
-    // TODO(weblayer-dev): Update when signin is supported.
-    return false;
-  }
+  // Signed-in state shouldn't control the set of variations for WebLayer,
+  // so this always returns true. This is particularly experiment for
+  // registering external experiment ids, which are registered assuming
+  // signed-in.
+  // TODO(sky): this is rather misleading, and needs to be resolved. Figure
+  // out right long term solution.
+  bool IsSignedIn() const { return true; }
 
   content::BrowserContext* browser_context_;
 };