content: Improve HostDiscardableSharedMemoryManager's memory pressure handling.

content/common/host_discardable_shared_memory_manager.cc

 // Copyright 2014 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 "content/common/host_discardable_shared_memory_manager.h"
 
 #include <algorithm>
 
 #include "base/callback.h"
 #include "base/debug/crash_logging.h"
@@ skipping 141 matching lines @@
   enforce_memory_policy_pending_ = false;
   ReduceMemoryUsageUntilWithinMemoryLimit();
 }
 
 void HostDiscardableSharedMemoryManager::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
   base::AutoLock lock(lock_);
 
   switch (memory_pressure_level) {
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE:
+      break;
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_MODERATE:
+      // Purge memory until usage is within half of |memory_limit_|.
+      ReduceMemoryUsageUntilWithinLimit(memory_limit_ / 2);
       break;
     case base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL:
       // Purge everything possible when pressure is critical.
       ReduceMemoryUsageUntilWithinLimit(0);
       break;
   }
 }
 
 void
 HostDiscardableSharedMemoryManager::ReduceMemoryUsageUntilWithinMemoryLimit() {
@@ skipping 74 matching lines @@
 
   enforce_memory_policy_pending_ = true;
   base::MessageLoop::current()->PostDelayedTask(
       FROM_HERE,
       base::Bind(&HostDiscardableSharedMemoryManager::EnforceMemoryPolicy,
                  weak_ptr_factory_.GetWeakPtr()),
       base::TimeDelta::FromMilliseconds(kEnforceMemoryPolicyDelayMs));
 }
 
 }  // namespace content