Add memory pressure notification API

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 62319203f418c4522b103b3d54ced4e43f09971b..1938da89e82cb1ddbf321a64994c73943c9d0b9b 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5511,6 +5511,14 @@ class V8_EXPORT Isolate {
   };
 
   /**
+   * Memory pressure level for the MemoryPressureNotification.
+   * kNone hints V8 that there is no memory pressure.
+   * kModerate hints V8 to speed up incremental garbage collection.
+   * kCritical hints V8 to free memory as soon as possible.
+   */
+  enum class MemoryPressureLevel { kNone, kModerate, kCritical };
+
+  /**
    * Features reported via the SetUseCounterCallback callback. Do not change
    * assigned numbers of existing items; add new features to the end of this
    * list.
@@ -5591,6 +5599,14 @@ class V8_EXPORT Isolate {
       AbortOnUncaughtExceptionCallback callback);
 
   /**
+   * Optional notification that the system is running low on memory.
+   * V8 uses these notifications to guide heuristics.
+   * It is allowed to call this function from another thread while
+   * the isolate is executing long running JavaScript code.

[jochen (gone - plz use gerrit), 2016/03/17 19:33:39]

do we expect the embedder to set the pressure level to kNone if the memory
pressure ends?

[ulan, 2016/03/18 12:49:40]

That it nice to have, but we do not rely on that. After GC we set the level to
kNone ourselves. If the embedder sets to kNone proactively before GC, then it
will be good for performance.

+   */
+  void MemoryPressureNotification(MemoryPressureLevel level);
+
+  /**
    * Methods below this point require holding a lock (using Locker) in
    * a multi-threaded environment.
    */