Defer the real work in updateCompositingLayers until it's really needed.

LayoutTests/http/tests/inspector/inspector-test.js

 var initialize_InspectorTest = function() {
 
 var results = [];
 var resultsSynchronized = false;
 
 function consoleOutputHook(messageType)
 {
     InspectorTest.addResult(messageType + ": " + Array.prototype.slice.call(arguments, 1));
 }
 
@@ skipping 462 matching lines @@
     setTimeout(step, 100);
 }
 
 function runTest(enableWatchDogWhileDebugging)
 {
     if (!window.testRunner)
         return;
 
     testRunner.dumpAsText();
     testRunner.waitUntilDone();
+    testRunner.display();

[caseq, 2013/10/17 12:37:38]

Why is this necessary? We used to invoke this only when this is actually
required. It's not that I oppose to doing this unconditionally, but it would be
nice to understand why this is required.

[shawnsingh, 2013/10/17 19:00:05]

Without testRunner.display() here, compositing was not getting updated.  As a
result, composited layers were not setup the way that tests were expecting.  A
lighter-weight option would be to use
window.internals.forceCompositingUpdate(document) but it seemed that testRunner
usage was the pattern in this test already.

 
     function initializeFrontend(initializationFunctions)
     {
         if (window.InspectorTest) {
             InspectorTest.pageLoaded();
             return;
         }
 
         InspectorTest = {};
     
@@ skipping 123 matching lines @@
     evaluateInWebInspector: function(callId, script)
     {
         window.opener.postMessage(["evaluateInWebInspector", callId, script], "*");
     },
 
     display: function() { }
 }
 
 if (!window.testRunner && window.opener)
     window.testRunner = new StandaloneTestRunnerStub();