Bug 1540856: Part 3 - Ensure the default theme is enabled for browser mochitests. r=aswan

Differential Revision: https://phabricator.services.mozilla.com/D25679

--HG--
extra : rebase_source : f8abe07c8d7457bf171f313eb2946d75da90269d
extra : histedit_source : 9c03cf7d02cbd1d19550770601dcd2a8b793418f

testing/mochitest/browser-test.js

@@ -12,6 +12,8 @@ var gSaveInstrumentationData = null;
 var {AppConstants} = ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
 var {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
 
+ChromeUtils.defineModuleGetter(this, "AddonManager",
+  "resource://gre/modules/AddonManager.jsm");
 ChromeUtils.defineModuleGetter(this, "ContentSearch",
   "resource:///modules/ContentSearch.jsm");
 
@@ -522,6 +524,7 @@ Tester.prototype = {
   },
 
   async waitForWindowsReady() {
+    await this.setupDefaultTheme();
     await new Promise(resolve => this.waitForGraphicsTestWindowToBeGone(resolve));
     await this.promiseMainWindowReady();
   },
@@ -533,6 +536,13 @@ Tester.prototype = {
     }
   },
 
+  async setupDefaultTheme() {
+    // Developer Edition enables the wrong theme by default. Make sure
+    // the ordinary default theme is enabled.
+    let theme = await AddonManager.getAddonByID("default-theme@mozilla.org");
+    await theme.enable();
+  },
+
   waitForGraphicsTestWindowToBeGone(aCallback) {
     for (let win of Services.wm.getEnumerator(null)) {
       if (win != window && !win.closed &&