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

public/platform/WebScheduler.h

 // Copyright 2014 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.
 
 #ifndef WebScheduler_h
 #define WebScheduler_h
 
 #include "WebCommon.h"
 #include "public/platform/WebThread.h"
 
@@ skipping 23 matching lines @@
     // and the caller should yield to let the scheduler service that work.
     // Must be called on the main thread.
     virtual bool shouldYieldForHighPriorityWork() { return false; }
 
     // Schedule an idle task to run the Blink main thread. For non-critical
     // tasks which may be reordered relative to other task types and may be
     // starved for an arbitrarily long time if no idle time is available.
     // 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.
     // Takes ownership of |IdleTask|. Can be called from any thread.
     virtual void postIdleTaskAfterWakeup(const WebTraceLocation&, IdleTask*) { }
 
     // Schedule a loading task to be run on the Blink main thread. Loading
     // tasks usually have the default priority, but may be deprioritised
     // when the user is interacting with the device.
     // Takes ownership of |WebThread::Task|. Can be called from any thread.
     virtual void postLoadingTask(const WebTraceLocation&, WebThread::Task*) { }
 };
 
 } // namespace blink
 
 #endif // WebScheduler_h