Adds MemoryPressureListener.

base/memory/memory_pressure_level_list.h

Index: base/memory/memory_pressure_level_list.h
diff --git a/base/memory/memory_pressure_level_list.h b/base/memory/memory_pressure_level_list.h
new file mode 100644
index 0000000000000000000000000000000000000000..d65e985a9021034562463da275146390090afab9
--- /dev/null
+++ b/base/memory/memory_pressure_level_list.h
@@ -0,0 +1,19 @@
+// 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_LEVEL
+#error "DEFINE_MEMORY_PRESSURE_LEVEL should be defined."
+#endif
+
+// Modules are advised to free buffers that are cheap to re-allocate and not
+// immediately needed.
+DEFINE_MEMORY_PRESSURE_LEVEL(MEMORY_PRESSURE_MODERATE, 0)
+
+// At this level, modules are advised to free all possible memory.
+// The alternative is to be killed by the system, which means all memory will
+// have to be re-created, plus the cost of a cold start.
+DEFINE_MEMORY_PRESSURE_LEVEL(MEMORY_PRESSURE_CRITICAL, 2)