base: Add soft memory limit to DiscardableMemoryManager.

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 9189b5eb027901a0e6e22d87853cd928b0c9fb4b..85a067f2f944774efa40527f853e85cfc8b25554 100644
--- a/base/memory/discardable_memory.h
+++ b/base/memory/discardable_memory.h
@@ -97,6 +97,13 @@ class BASE_EXPORT DiscardableMemory {
   // Create a DiscardableMemory instance with preferred type and |size|.
   static scoped_ptr<DiscardableMemory> CreateLockedMemory(size_t size);
 
+  // Call this when idle. Discardable memory implementations might use this
+  // to reduce memory footprint. Returns true if there's no need to call this
+  // again until memory instances have been used. This indicates that all
+  // discardable memory implementations have done as much cleanup as they
+  // will be able to do.
+  static bool IdleNotification();
+
   // Locks the memory so that it will not be purged by the system. Returns
   // DISCARDABLE_MEMORY_LOCK_STATUS_SUCCESS on success. If the return value is
   // DISCARDABLE_MEMORY_LOCK_STATUS_FAILED then this object should be