workers: Move where idle-tasks are turned on for v8::Isolate.

Compositor-workers currently do not support idle tasks (and so does not
have idle-task-runner). So idle-tasks should not be turned on for the
compositor workers' isolate. To fix this, move the code that turns on
idle tasks for the isolate into WorkerThread::initializeIsolate(), so
that compositor worker can choose to not do this for now.

BUG=565789, 430155

Review URL: https://codereview.chromium.org/1612093003

Cr-Commit-Position: refs/heads/master@{#371656}

third_party/WebKit/Source/core/workers/WorkerThread.cpp

@@ -287,9 +287,6 @@ void WorkerThread::initialize(PassOwnPtr<WorkerThreadStartupData> startupData)
         backingThread().addTaskObserver(m_microtaskRunner.get());
 
         m_isolate = initializeIsolate();
-        if (RuntimeEnabledFeatures::v8IdleTasksEnabled()) {
-            V8PerIsolateData::enableIdleTasks(m_isolate, adoptPtr(new V8IdleTaskRunner(m_webScheduler)));
-        }
         // Optimize for memory usage instead of latency for the worker isolate.
         m_isolate->IsolateInBackgroundNotification();
         m_workerGlobalScope = createWorkerGlobalScope(startupData);
@@ -487,7 +484,8 @@ v8::Isolate* WorkerThread::initializeIsolate()
     OwnPtr<V8IsolateInterruptor> interruptor = adoptPtr(new V8IsolateInterruptor(isolate));
     ThreadState::current()->addInterruptor(interruptor.release());
     ThreadState::current()->registerTraceDOMWrappers(isolate, V8GCController::traceDOMWrappers);
-
+    if (RuntimeEnabledFeatures::v8IdleTasksEnabled())
+        V8PerIsolateData::enableIdleTasks(isolate, adoptPtr(new V8IdleTaskRunner(m_webScheduler)));
     return isolate;
 }