Update from https://crrev.com/316786

base/memory/discardable_memory_shmem_allocator.cc

Index: base/memory/discardable_memory_shmem_allocator.cc
diff --git a/base/memory/discardable_memory_shmem_allocator.cc b/base/memory/discardable_memory_shmem_allocator.cc
index 8abe4569382e8d66f407aba397f296fbd6e17b62..761204f31220d043e526f6cb35e3548e6eca237f 100644
--- a/base/memory/discardable_memory_shmem_allocator.cc
+++ b/base/memory/discardable_memory_shmem_allocator.cc
@@ -18,16 +18,12 @@ class DiscardableMemoryShmemChunkImpl : public DiscardableMemoryShmemChunk {
       : shared_memory_(shared_memory.Pass()) {}
 
   // Overridden from DiscardableMemoryShmemChunk:
-  bool Lock() override {
-    auto result = shared_memory_->Lock(0, 0);
-    DCHECK_NE(result, DiscardableSharedMemory::PURGED);
-    return result == DiscardableSharedMemory::SUCCESS;
+  bool Lock() override { return false; }
+  void Unlock() override {
+    shared_memory_->Unlock(0, 0);
+    shared_memory_.reset();
   }
-  void Unlock() override { shared_memory_->Unlock(0, 0); }
   void* Memory() const override { return shared_memory_->memory(); }
-  bool IsMemoryResident() const override {
-    return shared_memory_->IsMemoryResident();
-  }
 
  private:
   scoped_ptr<DiscardableSharedMemory> shared_memory_;