Bug 1453519 - Avoid process reselection inside RDM. r=ochameau

Use `E10SUtils` to check for cases where browser code would try to force a
browser to reselect a new process (such as when it is a preloaded browser) so
that we can avoid any process changes once RDM is open.

If such a case applies, navigate to about:blank first to trigger the process
change before starting RDM.

MozReview-Commit-ID: CxspLFXXotF

--HG--
extra : rebase_source : cb94789344f17c0b51ff4afbb54921218a2dd1b5

devtools/client/responsive.html/browser/swap.js

@@ -5,6 +5,7 @@
 "use strict";
 
 const { Ci } = require("chrome");
+const { E10SUtils } = require("resource://gre/modules/E10SUtils.jsm");
 const { tunnelToInnerBrowser } = require("./tunnel");
 
 function debug(msg) {
@@ -89,9 +90,32 @@ function swapToInnerBrowser({ tab, containerURL, getInnerBrowser }) {
     }
   };
 
+  // Wait for a browser to load into a new frame loader.
+  function loadURIWithNewFrameLoader(browser, uri, options) {
+    return new Promise(resolve => {
+      gBrowser.addEventListener("XULFrameLoaderCreated", resolve, { once: true });
+      browser.loadURI(uri, options);
+    });
+  }
+
   return {
 
     async start() {
+      // In some cases, such as a preloaded browser used for about:newtab, browser code
+      // will force a new frameloader on next navigation to ensure balanced process
+      // assignment.  If this case will happen here, navigate to about:blank first to get
+      // this out of way so that we stay within one process while RDM is open.
+      let { newFrameloader } = E10SUtils.shouldLoadURIInBrowser(
+        tab.linkedBrowser,
+        "about:blank"
+      );
+      if (newFrameloader) {
+        debug(`Tab will force a new frameloader on navigation, load about:blank first`);
+        await loadURIWithNewFrameLoader(tab.linkedBrowser, "about:blank", {
+          flags: Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_HISTORY,
+        });
+      }
+
       tab.isResponsiveDesignMode = true;
 
       // Hide the browser content temporarily while things move around to avoid displaying