MD Settings: Adding some unit tests for <settings-main>.

chrome/browser/resources/settings/settings_page/main_page_behavior.js

Index: chrome/browser/resources/settings/settings_page/main_page_behavior.js
diff --git a/chrome/browser/resources/settings/settings_page/main_page_behavior.js b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
index 933a0ec4baab3a6cd5c57404141fed70b549de6c..03fb03735ade072b0828ce4297a715a22258a0de 100644
--- a/chrome/browser/resources/settings/settings_page/main_page_behavior.js
+++ b/chrome/browser/resources/settings/settings_page/main_page_behavior.js
@@ -41,7 +41,10 @@ var MainPageBehaviorImpl = {
 
   /** @override */
   attached: function() {
-    this.scroller = this.domHost && this.domHost.parentNode.$.mainContainer;
+    if (this.domHost && this.domHost.parentNode.tagName == 'PAPER-HEADER-PANEL')
+      this.scroller = this.domHost.parentNode.$.mainContainer;
+    else
+      this.scroller = document.body; // Used in unit tests.

[michaelpg, 2016/07/30 05:38:44]

could this just be assert(this.parentElement)? if we're attached but we're not
in a shadow tree we must be a normal DOM child.

it can equate to document.body in tests, but it also means we could parent this
thing with a <div> to test the scrolling behaviors without messing with the
entire document.body.

[michaelpg, 2016/08/02 20:45:20]

or better, see my new suggestion

[dpapad, 2016/08/02 22:33:15]

Previous suggestion does not work.
this.parentElement is null at the time the attached() callback fires.
this.parentNode is [object ShadowRoot] (still causes tests to fail).

I would rather not spend too much time tweaking this line. Instead the root
issue https://bugs.chromium.org/p/chromium/issues/detail?id=631281 should be
addressed. See also my response to your newest suggestion.

   },
 
   /**