Update from chromium https://crrev.com/301315

base/memory/discardable_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 "base/memory/discardable_memory_manager.h"
 
 #include "base/bind.h"
 #include "base/containers/adapters.h"
 #include "base/containers/hash_tables.h"
 #include "base/containers/mru_cache.h"
@@ skipping 33 matching lines @@
   AutoLock lock(lock_);
   soft_memory_limit_ = bytes;
 }
 
 void DiscardableMemoryManager::SetHardMemoryLimitExpirationTime(
     TimeDelta hard_memory_limit_expiration_time) {
   AutoLock lock(lock_);
   hard_memory_limit_expiration_time_ = hard_memory_limit_expiration_time;
 }
 
+void DiscardableMemoryManager::ReleaseFreeMemory() {
+  TRACE_EVENT0("base", "DiscardableMemoryManager::ReleaseFreeMemory");
+
+  AutoLock lock(lock_);
+  size_t bytes_allocated_before_releasing_memory = bytes_allocated_;
+  for (auto& entry : allocations_) {
+    Allocation* allocation = entry.first;
+    AllocationInfo* info = &entry.second;
+
+    if (!info->purgable)
+      continue;
+
+    // Skip if memory is still resident, otherwise purge and adjust
+    // |bytes_allocated_|.
+    if (allocation->IsMemoryResident())
+      continue;
+
+    size_t bytes_purgable = info->bytes;
+    DCHECK_LE(bytes_purgable, bytes_allocated_);
+    bytes_allocated_ -= bytes_purgable;
+    info->purgable = false;
+    allocation->Purge();
+  }
+
+  if (bytes_allocated_ != bytes_allocated_before_releasing_memory)
+    BytesAllocatedChanged(bytes_allocated_);
+}
+
 bool DiscardableMemoryManager::ReduceMemoryUsage() {
   return PurgeIfNotUsedSinceHardLimitCutoffUntilWithinSoftMemoryLimit();
 }
 
 void DiscardableMemoryManager::ReduceMemoryUsageUntilWithinLimit(size_t bytes) {
   AutoLock lock(lock_);
   PurgeIfNotUsedSinceTimestampUntilUsageIsWithinLimitWithLockAcquired(Now(),
                                                                       bytes);
 }
 
@@ skipping 145 matching lines @@
   base::debug::SetCrashKeyValue(kDiscardableMemoryUsageKey,
                                 Uint64ToString(new_bytes_allocated));
 }
 
 TimeTicks DiscardableMemoryManager::Now() const {
   return TimeTicks::Now();
 }
 
 }  // namespace internal
 }  // namespace base