Bug 1619992 - Split AboutNewTabService into AboutNewTabParentService and AboutNewTabChildService. r=perftest-reviewers,Mardak,sparky

This patch does the following:
* Moves most logic for initiating about:home / about:newtab into AboutNewTab.jsm
* Makes AboutNewTab the API surface for overriding the default about:newtab URLs.
* Reduces the surface of nsIAboutNewTabService, and makes the properties read-only
* Splits the remaining code in the nsIAboutNewTabService into an implementation for
  the parent process, and one for content processes.

This split will hopefully help reduce confusion about which code in
AboutNewTabService is running in which process.

Differential Revision: https://phabricator.services.mozilla.com/D65569

browser/base/content/utilityOverlay.js

@@ -13,6 +13,7 @@ var { XPCOMUtils } = ChromeUtils.import(
 );
 
 XPCOMUtils.defineLazyModuleGetters(this, {
+  AboutNewTab: "resource:///modules/AboutNewTab.jsm",
   BrowserWindowTracker: "resource:///modules/BrowserWindowTracker.jsm",
   ContextualIdentityService:
     "resource://gre/modules/ContextualIdentityService.jsm",
@@ -21,13 +22,6 @@ XPCOMUtils.defineLazyModuleGetters(this, {
   ShellService: "resource:///modules/ShellService.jsm",
 });
 
-XPCOMUtils.defineLazyServiceGetter(
-  this,
-  "aboutNewTabService",
-  "@mozilla.org/browser/aboutnewtab-service;1",
-  "nsIAboutNewTabService"
-);
-
 XPCOMUtils.defineLazyGetter(this, "ReferrerInfo", () =>
   Components.Constructor(
     "@mozilla.org/referrer-info;1",
@@ -42,7 +36,7 @@ Object.defineProperty(this, "BROWSER_NEW_TAB_URL", {
     if (PrivateBrowsingUtils.isWindowPrivate(window)) {
       if (
         !PrivateBrowsingUtils.permanentPrivateBrowsing &&
-        !aboutNewTabService.overridden
+        !AboutNewTab.newTabURLOverridden
       ) {
         return "about:privatebrowsing";
       }
@@ -61,12 +55,12 @@ Object.defineProperty(this, "BROWSER_NEW_TAB_URL", {
       if (
         !privateAllowed &&
         (extensionControlled ||
-          aboutNewTabService.newTabURL.startsWith("moz-extension://"))
+          AboutNewTab.newTabURL.startsWith("moz-extension://"))
       ) {
         return "about:privatebrowsing";
       }
     }
-    return aboutNewTabService.newTabURL;
+    return AboutNewTab.newTabURL;
   },
 });
 
@@ -655,7 +649,7 @@ function openLinkIn(url, where, params) {
       focusUrlBar =
         !loadInBackground &&
         w.isBlankPageURL(url) &&
-        !aboutNewTabService.willNotifyUser;
+        !AboutNewTab.willNotifyUser;
 
       let tabUsedForLoad = w.gBrowser.loadOneTab(url, {
         referrerInfo: aReferrerInfo,