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

public/platform/WebScheduler.h

Index: public/platform/WebScheduler.h
diff --git a/public/platform/WebScheduler.h b/public/platform/WebScheduler.h
index a2cda3eac91603b1f741794377d01c942d88d16e..250d5ccf2fe03606803974cbd1a3f60a13f3be41 100644
--- a/public/platform/WebScheduler.h
+++ b/public/platform/WebScheduler.h
@@ -41,6 +41,13 @@ public:
     // Takes ownership of |IdleTask|. Can be called from any thread.
     virtual void postIdleTask(const WebTraceLocation&, 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.
+    virtual void postNonNestableIdleTask(const WebTraceLocation&, 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.