ios: Use webkit.postMessage to flush IPC during session restore.

Session restore uses history.pushState to inject URLs and titles into
each WKWebView session.  For large numbers of pages, this can generate
a very large number of IPC calls. iOS 12.2 introduced IPC throttling
that can delay the async pushState calls before the sync history.go
call. This was worked around via a check to see when all the push state
calls are correctly processed.

iOS14 introduced an optimization which breaks the above mentioned
workaround.  Instead, this change uses our own IPC call to native code
(which will be enqueued to the end of the IPC queue) to finalize session
restore.

The effect is faster restore for the average case, but slightly slower
restore for the edge case of a large numbers of pages.

Bug: 1127521, 1126863
Change-Id: Ibb70008afef312897e8165e06f16ed5c03808b38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407332
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806721}

ios/web/navigation/resources/restore_session.html

@@ -90,24 +90,34 @@
               getRestoreURL(sessionHistoryObject.urls[i] || "about:blank"));
         }
 
-        // iOS12.2 added a throttling mechanism where previous pushStates may
-        // not immediately be available. Set a 10ms interval delay until
-        // history.length reaches sessionHistory.length.
+        // iOS12.2 added a throttling mechanism where the previous pushStates
+        // may not immediately be available.  Instead of directly calling
+        // finishRestore(currentItemOffset) here, call it via a round trip
+        // native call to flush out the pushState IPC backlog.
         var currentItemOffset = parseInt(sessionHistoryObject.offset);
-        var goWhenReady = setInterval(() => {
-          if (history.length == sessionHistoryObject.urls.length) {
-            history.go(currentItemOffset);
-            window.clearInterval(goWhenReady);
+        var offset = {'offset': currentItemOffset}
+        window.webkit.messageHandlers['session_restore'].postMessage(offset);
+       } catch (e) {
+         handleError(e.name + ": " + e.message + " raw session history: " + sessionHistory);
+       }
+     }
 
-            // Queue a reload to redirect to the target URL after history.go
-            // is processed.
-            setTimeout(() => {
-              location.reload();
-            });
-           }
-        }, 10);
+    /**
+     * Finishes restoration by history.go-ing to |offset| and reloading the
+     * page.
+     * @param {int} offset Either zero, or the number of pages (negative) to
+     * go backwards.
+     */
+    function _crFinishSessionRestoration(offset) {
+      try {
+        history.go(offset);
+        // Queue a reload to redirect to the target URL after history.go
+        // is processed.
+        setTimeout(() => {
+          location.reload();
+        });
       } catch (e) {
-        handleError(e.name + ": " + e.message + " raw session history: " + sessionHistory);
+        handleError(e.name + ": " + e.message + " offset: " + offset);
       }
     }
 

ios/web/web_state/ui/crw_web_controller.mm

@@ -86,6 +86,9 @@ NSString* const kIsMainFrame = @"isMainFrame";
 // URL scheme for messages sent from javascript for asynchronous processing.
 NSString* const kScriptMessageName = @"crwebinvoke";
 
+// URL scheme for session restore.
+NSString* const kSessionRestoreScriptMessageName = @"session_restore";
+
 }  // namespace
 
 @interface CRWWebController () <CRWWKNavigationHandlerDelegate,
@@ -443,6 +446,15 @@ typedef void (^ViewportStateCompletion)(const web::PageViewportState*);
                            name:kScriptMessageName
                         webView:_webView];
 
+    // TODO(crbug.com/1127521) Consider consolidating session restore script
+    // logic into a different place.
+    [messageRouter
+        setScriptMessageHandler:^(WKScriptMessage* message) {
+          [weakSelf didReceiveSessionRestoreScriptMessage:message];
+        }
+                           name:kSessionRestoreScriptMessageName
+                        webView:_webView];
+
     _webView.allowsBackForwardNavigationGestures =
         _allowsBackForwardNavigationGestures;
   }
@@ -1105,6 +1117,22 @@ typedef void (^ViewportStateCompletion)(const web::PageViewportState*);
   }
 }
 
+// TODO(crbug.com/1127521) Consider consolidating session restore script
+// logic into a different place.
+- (void)didReceiveSessionRestoreScriptMessage:(WKScriptMessage*)message {
+  if ([message.name isEqualToString:kSessionRestoreScriptMessageName] &&
+      [message.body[@"offset"] isKindOfClass:[NSNumber class]]) {
+    NSString* method =
+        [NSString stringWithFormat:@"_crFinishSessionRestoration('%@')",
+                                   message.body[@"offset"]];
+    // Don't use |_jsInjector| -executeJavaScript here, as it relies on
+    // |windowID| being injected before window.onload starts.
+    web::ExecuteJavaScript(self.webView, method, nil);
+  } else {
+    DLOG(WARNING) << "Invalid session restore JS message name.";
+  }
+}
+
 - (BOOL)respondToWKScriptMessage:(WKScriptMessage*)scriptMessage {
   if (![scriptMessage.name isEqualToString:kScriptMessageName]) {
     return NO;