Modifies few gc related layout tests to work with ignition.

third_party/WebKit/LayoutTests/fast/dom/MutationObserver/weak-callback-gc-crash.html

Index: third_party/WebKit/LayoutTests/fast/dom/MutationObserver/weak-callback-gc-crash.html
diff --git a/third_party/WebKit/LayoutTests/fast/dom/MutationObserver/weak-callback-gc-crash.html b/third_party/WebKit/LayoutTests/fast/dom/MutationObserver/weak-callback-gc-crash.html
index 4cb6819f14bb9594483aaa5efd7575a9f78e152a..fe413c9f08dab257c6eca00aa26ad7128e9a0332 100644
--- a/third_party/WebKit/LayoutTests/fast/dom/MutationObserver/weak-callback-gc-crash.html
+++ b/third_party/WebKit/LayoutTests/fast/dom/MutationObserver/weak-callback-gc-crash.html
@@ -5,11 +5,20 @@
 if (window.testRunner) {
     testRunner.waitUntilDone();
     testRunner.dumpAsText();
+} 
+var observer, div;
+
+function initializeObserver() {
+    observer = new MutationObserver(
+                   function() {console.log('Should not appear')});
+    div = document.createElement('div');
+    observer.observe(div, {attributes: true});
+    div.id = 'foo';
 }
-var observer = new MutationObserver(function() {console.log('Should not appear')});
-var div = document.createElement('div');
-observer.observe(div, {attributes: true});
-div.id = 'foo';
+
+// Perform all assignments in a function to avoid references to objects
+// in dead registers when using ignition (chromium:595672)

[rmcilroy, 2016/05/13 11:29:37]

/s/Perform all assignments in a function/Do initialization work in an inner
function/
/s/in dead registers/remaining live on this function's stack frame/
/s/(chromium:595672)/ - http://crbug.com/595672/ 

And thoughout.

[mythria, 2016/05/16 11:15:10]

Done.

+initializeObserver();
 div = null;
 observer = null;
 gc();