Add support for postNonNestableIdleTask to the Scheduler and use it in Oilpan

Source/platform/scheduler/Scheduler.h

Index: Source/platform/scheduler/Scheduler.h
diff --git a/Source/platform/scheduler/Scheduler.h b/Source/platform/scheduler/Scheduler.h
index 2a7d84b268c3f5ea7800edee3e816689b35412a9..f516d9e6e74bc083fdf025ac3d8382fec711a90c 100644
--- a/Source/platform/scheduler/Scheduler.h
+++ b/Source/platform/scheduler/Scheduler.h
@@ -31,6 +31,13 @@ public:
     // for an arbitrarily long time if no idle time is available.
     void postIdleTask(const WebTraceLocation&, PassOwnPtr<IdleTask>);
 
+    // Like postIdleTask but guarantees that the posted task will not run
+    // nested within an already-running task. Posting an idle task as
+    // non-nestable may not affect when the task gets run, or it could
+    // make it run later than it normally would, but it won't make it
+    // run earlier than it normally would.
+    void postNonNestableIdleTask(const WebTraceLocation&, PassOwnPtr<IdleTask>);
+
     // Like postIdleTask but does not run the idle task until after some other
     // task has run. This enables posting of a task which won't stop the Blink
     // main thread from sleeping, but will start running after it wakes up.