Bug 802881 - Use correct tab for getting background color. r=kats

mobile/android/chrome/content/browser.js

@@ -2800,12 +2800,9 @@ Tab.prototype = {
         // event fires; it's not clear that doing so is worth the effort.
         var backgroundColor = null;
         try {
-          let browser = BrowserApp.selectedBrowser;
-          if (browser) {
-            let { contentDocument, contentWindow } = browser;
-            let computedStyle = contentWindow.getComputedStyle(contentDocument.body);
-            backgroundColor = computedStyle.backgroundColor;
-          }
+          let { contentDocument, contentWindow } = this.browser;
+          let computedStyle = contentWindow.getComputedStyle(contentDocument.body);
+          backgroundColor = computedStyle.backgroundColor;
         } catch (e) {
           // Ignore. Catching and ignoring exceptions here ensures that Talos succeeds.
         }