Show a warning message if the cache might not be cleared correctly between runs.

chrome/common/extensions/docs/examples/extensions/benchmark/background.html

 <style>
 #options {
   position: absolute;
   background-color: #FFFFCC;
   display: none;
   font-family: "Courier New";
   font-size: 9pt;
   padding: 5px;
   border: 1px solid #CCCC88;
   z-index: 3;
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     if (data.message == "load") {
       var benchmark = findBenchmark(data.url);
       if (benchmark != undefined && benchmark.isRunning()) {
         benchmark.pageFinished(data.values);
       }
     }
   });
 });
 
 function run() {
+  if (window.clearCache) {
+    // Show a warning if we will try to clear the cache between runs
+    // but will also be reusing the same WebKit instance (i.e. Chrome
+    // is in single-process mode, or 'Use New Tabs' is turned off)
+    // because the WebKit cache might not get completely cleared between runs.
+    if (chrome.benchmarking.isSingleProcess()) {
+      alert("Warning: the WebKit cache may not be cleared correctly " +
+            "between runs because Chrome is running in single-process mode.");
+    } else if (!window.useNewTabs) {
+      alert("Warning: the WebKit cache may not be cleared correctly " +
+            "between runs because 'Use New Tabs Per Page' is turned off.");
+    }
+  }
+
   var urls = testUrl.split(",");
   for (var i = 0; i < urls.length; i++) {
     var benchmark = new Benchmark();
     benchmark.start(urls[i]);  // XXXMB - move to constructor
   }
   benchmarks[0].runPage();
 }
 
 // Run at startup
 chrome.windows.getCurrent(function(currentWindow) {
   window.windowId = currentWindow.id;
 });
 </script>