Modified two extension tests to not be dependent on low-latency DOM timers

ExtensionApiTest.Bookmarks
 was adding one bookmark per timer callback; changed to use an event test.
WindowOpenPanelTest.WindowOpenPanel was relying on the callback supplied to chrome.windows.create happening before the resource was actually loaded, when there's no such guarantee.

R=kalman
BUG=424661,253417,383452

Review URL: https://codereview.chromium.org/1094383005

Cr-Commit-Position: refs/heads/master@{#326933}

chrome/test/data/extensions/api_test/bookmarks/manifest.json

@@ -4,6 +4,7 @@
   "manifest_version": 2,
   "description": "end-to-end browser test for chrome.bookmarks API",
   "background": {
+    "persistent": false,
     "scripts": ["test.js"]
   },
   "permissions": ["bookmarks"]

chrome/test/data/extensions/api_test/window_open/panel/test.js

@@ -3,9 +3,19 @@
 // found in the LICENSE file.
 
 var panelWindowId = 0;
+var panelLoaded = false;
 
 // This function is called by the panel during the test run.
 function panelCallback() {
+  panelLoaded = true;
+  maybeReadyForTest();
+}
+
+function maybeReadyForTest() {
+  // The order of the two callbacks is not guaranteed.
+  if( panelWindowId === 0 || !panelLoaded)
+    return;
+
   // We have now added a panel so the total counts is 2 (browser + panel).
   chrome.test.assertEq(2, chrome.extension.getViews().length);
   // Verify that we're able to get the view of the panel by its window id.
@@ -29,6 +39,7 @@ chrome.test.runTests([
             chrome.test.assertEq(true, win.alwaysOnTop);
             panelWindowId = win.id;
             // The panel will call back to us through panelCallback (above).
+            maybeReadyForTest();
         });
   }
 ]);