Adds MemoryPressureListener.

base/memory/memory_pressure_handler_list.h

Index: base/memory/memory_pressure_handler_list.h
diff --git a/base/memory/memory_pressure_handler_list.h b/base/memory/memory_pressure_handler_list.h
new file mode 100644
index 0000000000000000000000000000000000000000..df8a46bbbc1d174bc53136782907ddfbcd5bdfb6
--- /dev/null
+++ b/base/memory/memory_pressure_handler_list.h
@@ -0,0 +1,14 @@
+// Copyright (c) 2013 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.
+
+// This file intentionally does not have header guards, it's included
+// inside a macro to generate enum and a java class for the values.
+
+#ifndef DEFINE_MEMORY_PRESSURE_TYPE
+#error "DEFINE_MEMORY_PRESSURE_TYPE should be defined."
+#endif
+
+DEFINE_MEMORY_PRESSURE_TYPE(MEMORY_PRESSURE_LOW, 0)
+DEFINE_MEMORY_PRESSURE_TYPE(MEMORY_PRESSURE_MODERATE, 1)

[joth, 2013/06/05 16:51:23]

these need some descriptions.
Tom's suggestions of time it takes to recreate in the bug is nice starting point
- although that depends on the foreground/background state too of the 'thing'
being dropped, and it may seem confusing to talk about that here too. (It leads
to the question of should this carry for/background state too, but that's domain
specific, not a global state, so we don't want it tied in here)

[bulach, 2013/06/05 19:02:11]

yeah, I'm ambivalent of these levels...
on one hand, it seems that having at least _two_ levels is better than none, so
individual modules can fine tune their decision.
otoh, it makes this more contentious... :)
I made it just two levels, with comments, let me know if you think it's enough.

background / foreground should be conveyed in some other mechanism, and each
module should query that to decide what to flush.

+DEFINE_MEMORY_PRESSURE_TYPE(MEMORY_PRESSURE_CRITICAL, 2)