Reland r68893 after fixing allowing blocking IO on shutdown.

chrome/browser/geolocation/geolocation_provider.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/geolocation/geolocation_provider.h"
 
 #include "base/singleton.h"
+#include "base/thread_restrictions.h"
 #include "chrome/browser/geolocation/location_arbitrator.h"
 
 // This class is guaranteed to outlive its internal thread, so ref counting
 // is not required.
 DISABLE_RUNNABLE_METHOD_REFCOUNT(GeolocationProvider);
 
 GeolocationProvider* GeolocationProvider::GetInstance() {
   return Singleton<GeolocationProvider>::get();
 }
 
@@ skipping 22 matching lines @@
 bool GeolocationProvider::RemoveObserver(GeolocationObserver* observer) {
   DCHECK(OnClientThread());
   size_t remove = observers_.erase(observer);
   OnObserversChanged();
   return remove > 0;
 }
 
 void GeolocationProvider::OnObserversChanged() {
   DCHECK(OnClientThread());
   if (observers_.empty()) {
+    // http://crbug.com/66077: This is a bug.  The geolocation thread may
+    // transitively (via other threads it joins) block on long-running tasks /
+    // IO.
+    base::ThreadRestrictions::ScopedAllowIO allow_io;
     Stop();
   } else {
     if (!IsRunning()) {
       Start();
       if (HasPermissionBeenGranted())
         InformProvidersPermissionGranted(most_recent_authorized_frame_);
     }
     // The high accuracy requirement may have changed.
     Task* task = NewRunnableMethod(this,
         &GeolocationProvider::SetObserverOptions,
@@ skipping 71 matching lines @@
   return most_recent_authorized_frame_.is_valid();
 }
 
 bool GeolocationProvider::OnClientThread() const {
   return client_loop_->BelongsToCurrentThread();
 }
 
 bool GeolocationProvider::OnGeolocationThread() const {
   return MessageLoop::current() == message_loop();
 }