Allow waiting in background Icing work

The HistoryReportJniBridge.addHistoricVisitsToUsageReportsBuffer() method
runs on a background thread, so it is fine to use base::WaitableEvent
in the underlying JNI functions without affecting responsiveness.

Bug: 943381
Change-Id: I85150cf9ec29de15b3375587b89ccae265490330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531029Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646684}

base/threading/thread_restrictions.h

@@ -168,6 +168,9 @@ class InFlightIO;
 namespace functions {
 class ExecScriptScopedAllowBaseSyncPrimitives;
 }
+namespace history_report {
+class HistoryReportJniBridge;
+}
 namespace gpu {
 class GpuChannelHost;
 }
@@ -390,6 +393,7 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitives {
   friend class content::ServiceWorkerContextClient;
   friend class content::SessionStorageDatabase;
   friend class functions::ExecScriptScopedAllowBaseSyncPrimitives;
+  friend class history_report::HistoryReportJniBridge;
   friend class internal::TaskTracker;
   friend class leveldb::LevelDBMojoProxy;
   friend class leveldb_env::DBTracker;

chrome/browser/android/history_report/history_report_jni_bridge.cc

@@ -183,6 +183,11 @@ void HistoryReportJniBridge::StartReporting() {
 jboolean HistoryReportJniBridge::AddHistoricVisitsToUsageReportsBuffer(
     JNIEnv* env,
     const JavaParamRef<jobject>& obj) {
+  DCHECK(!content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  // Waiting is okay here because this is called from a background thread
+  // in Java.
+  base::ScopedAllowBaseSyncPrimitives allow_sync;
+
   data_provider_->StartVisitMigrationToUsageBuffer(
       usage_reports_buffer_service_.get());
   // TODO(nileshagrawal): Return true when actually done,