hasDescendantWithBlendMode should not be propagated outside the current stacking context.

PerformanceTests/Blending/resources/framerate.js

 (function(){
 
 var framesPerTimerReading = 10;
 var frameCount = 0;
 var startTime;
 var trackingFrameRate = false;
+var currentTest;
 
 function trackFrameRate(currTime)
 {
     if (++frameCount == framesPerTimerReading) {
         frameCount = 0;
         PerfTestRunner.measureValueAsync(1000 * framesPerTimerReading / (currTime - startTime));
         startTime = currTime;
     }
 
+    if (currentTest && currentTest.run)
+        currentTest.run();
+
     if (trackingFrameRate)
         requestAnimationFrame(trackFrameRate);
 }
 
-window.startTrackingFrameRate = function() {
+window.startTrackingFrameRate = function(test) {
     if (trackingFrameRate)
         return;
     trackingFrameRate = true;
+    currentTest = test;
     startTime = performance.now();
     trackFrameRate();
 };
 
 window.stopTrackingFrameRate = function() {
     trackingFrameRate = false;
+    currentTest = undefined;
 };
 
 })();