Check for warnings when loading extensions in browser tests.

chrome/test/data/extensions/api_test/canvas_2d/background.js

+canvas = document.getElementById("my-canvas");
+if (canvas) {
+  if (canvas.getContext) {
+    context = canvas.getContext("2d");
+    if (context) {
+      context.fillStyle = 'red';
+      context.fillRect(20, 20, 40, 40);
+      chrome.test.notifyPass();
+    } else {
+      chrome.test.notifyFail("unable to getContext('2d')");
+    }
+  } else {
+    chrome.test.notifyFail("canvas.getContext null");
+  }
+} else {
+  chrome.test.notifyFail("couldn't find element my-canvas");
+}