Fix flaky brailleDisplayPrivateApi tests and reenable them.

chrome/test/data/extensions/api_test/braille_display_private/write_dots/test.js

Index: chrome/test/data/extensions/api_test/braille_display_private/write_dots/test.js
diff --git a/chrome/test/data/extensions/api_test/braille_display_private/write_dots/test.js b/chrome/test/data/extensions/api_test/braille_display_private/write_dots/test.js
index 7f0d5a2a3b5676331ec2234ec9e3b85b2a457ad2..7254cf4810c2618a2245f69f30f97736b8a96d89 100644
--- a/chrome/test/data/extensions/api_test/braille_display_private/write_dots/test.js
+++ b/chrome/test/data/extensions/api_test/braille_display_private/write_dots/test.js
@@ -18,12 +18,18 @@ function createBuffer(size, element) {
 function waitForDisplay(callback) {
   var callbackCompleted = chrome.test.callbackAdded();
   var displayStateHandler = function(state) {
+    if (!callbackCompleted) {
+      return;
+    }
     chrome.test.assertTrue(state.available, "Display not available");
     chrome.test.assertEq(11, state.textCellCount);
     callback(state);
     callbackCompleted();
     chrome.brailleDisplayPrivate.onDisplayStateChanged.removeListener(
         displayStateHandler);
+    // Prevent additional runs if the onDisplayStateChanged event
+    // is fired before getDisplayState invokes the callback.
+    callbackCompleted = null;
   };
   chrome.brailleDisplayPrivate.onDisplayStateChanged.addListener(
       displayStateHandler);