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

[willchan no longer on Chromium, 2014/06/27 14:12:56]

I'd give more guidance on "idle". Specifically, discuss the tradeoffs. Calling
this too often will hurt performance, whereas calling it too infrequently will
bloat memory usage. At least, that's how I understand it :)

Actually, perhaps we should rename this. This function name assumes too much
about how the caller might want to use it. Yes, most likely the sane way to use
this is in an idle handler, but functionally what it does is try to discard
memory up to a threshold.

[reveman, 2014/06/27 20:38:02]

Correct. I've updated the comment.
Good idea. I changed the name to ReduceMemoryUsage and made it clear in the
comment what this function might do.

+  // 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