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

base/memory/discardable_memory.h

Index: base/memory/discardable_memory.h
diff --git a/base/memory/discardable_memory.h b/base/memory/discardable_memory.h
index 5f83e333775b664c9f1e796e9b372c22c1768888..d8b7a58fbbb60ac23517170c949d5c476868bea0 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -19,7 +19,8 @@ enum DiscardableMemoryType {
   DISCARDABLE_MEMORY_TYPE_NONE,
   DISCARDABLE_MEMORY_TYPE_ASHMEM,
   DISCARDABLE_MEMORY_TYPE_MACH,
-  DISCARDABLE_MEMORY_TYPE_EMULATED
+  DISCARDABLE_MEMORY_TYPE_EMULATED,
+  DISCARDABLE_MEMORY_TYPE_SHMEM
 };
 
 enum DiscardableMemoryLockStatus {
@@ -88,6 +89,10 @@ class BASE_EXPORT DiscardableMemory {
   // Create a DiscardableMemory instance with preferred type and |size|.
   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
 
+  // Discardable memory implementations might use this to release memory
+  // or resources assigned to instances that have been purged.
+  static void ReleaseFreeMemory();
+
   // Discardable memory implementations might allow an elevated usage level
   // while in frequent use. Call this to have the usage reduced to the base
   // level. Returns true if there's no need to call this again until