Bug 1706445 - Stop using "SessionStore:flush" for SHIP, r=nika

Differential Revision: https://phabricator.services.mozilla.com/D116144

browser/components/sessionstore/TabStateFlusher.jsm

@@ -6,6 +6,11 @@
 
 var EXPORTED_SYMBOLS = ["TabStateFlusher"];
 
+ChromeUtils.defineModuleGetter(
+  this,
+  "Services",
+  "resource://gre/modules/Services.jsm"
+);
 ChromeUtils.defineModuleGetter(
   this,
   "SessionStore",
@@ -110,6 +115,8 @@ var TabStateFlusherInternal = {
       */
       nativePromise = browser.frameLoader.requestTabStateFlush();
     }
+
+    if (!Services.appinfo.sessionHistoryInParent) {
       /*
         In the event that we have to trigger a process switch and thus change
         browser remoteness, session store needs to register and track the new
@@ -125,6 +132,7 @@ var TabStateFlusherInternal = {
         id,
         epoch: SessionStore.getCurrentEpoch(browser),
       });
+    }
 
     // Retrieve active requests for given browser.
     let permanentKey = browser.permanentKey;
@@ -136,13 +144,20 @@ var TabStateFlusherInternal = {
       this._requests.set(permanentKey, request);
     }
 
-    let promise = new Promise(resolve => {
+    // Non-SHIP flushes resolve this after the "SessionStore:update" message. We
+    // don't use that message for SHIP, so it's fine to resolve the request
+    // immediately after the native promise resolves, since SessionStore will
+    // have processed all updates from this browser by that point.
+    let requestPromise = Promise.resolve();
+    if (!Services.appinfo.sessionHistoryInParent) {
+      requestPromise = new Promise(resolve => {
         // Store resolve() so that we can resolve the promise later.
         request.perBrowserRequests.set(id, resolve);
       });
+    }
 
     return Promise.race([
-      nativePromise.then(_ => promise),
+      nativePromise.then(_ => requestPromise),
       request.cancelPromise,
     ]);
   },