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

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

Index: LayoutTests/http/tests/inspector/inspector-test.js
diff --git a/LayoutTests/http/tests/inspector/inspector-test.js b/LayoutTests/http/tests/inspector/inspector-test.js
index b581a822999568334a6482ea29fd727c44491a86..08ba0a6714473e99cab28e163b3febef0662b9eb 100644
--- a/LayoutTests/http/tests/inspector/inspector-test.js
+++ b/LayoutTests/http/tests/inspector/inspector-test.js
@@ -480,6 +480,7 @@ function runTest(enableWatchDogWhileDebugging)
 
     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)
     {