Add trace event to sync_sessions::OnReadAllMetadata()

It is likely a cause of janks on UI thread on Android.
Add a trace event to get metrics about the duration.

BUG=902203

Change-Id: I4c4e9c2a20790264b982007ea7ee88ddfa7b972c
Reviewed-on: https://chromium-review.googlesource.com/c/1319369Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606104}

components/sync_sessions/session_store.cc

@@ -16,6 +16,7 @@
 #include "base/metrics/histogram_macros.h"
 #include "base/pickle.h"
 #include "base/strings/stringprintf.h"
+#include "base/trace_event/trace_event.h"
 #include "components/sync/base/time.h"
 #include "components/sync/device_info/device_info.h"
 #include "components/sync/device_info/device_info_util.h"
@@ -172,6 +173,8 @@ class FactoryImpl : public base::SupportsWeakPtr<FactoryImpl> {
       std::unique_ptr<ModelTypeStore::RecordList> record_list,
       const base::Optional<syncer::ModelError>& error,
       std::unique_ptr<syncer::MetadataBatch> metadata_batch) {
+    // Remove after fixing https://crbug.com/902203.
+    TRACE_EVENT0("browser", "FactoryImpl::OnReadAllMetadata");
     if (error) {
       std::move(callback).Run(error, /*store=*/nullptr,
                               /*metadata_batch=*/nullptr);