[Nav Experiment] Support async loading in PurgeCachedWebViewPages().

PurgeCachedWebViews() restores session history and wait for the first
navigation after restore to stop loading before returning. However, if
the first loading event happens asynchronously in the web view,
PurgeCachedWebViews() would return before session restore is finished.
This CL changes it to wait for the navigation finish event to handle
this situation.

This fixes NavigationTestCase//testRestoreHistoryToNTPAndNavigateForward
when #slim-navigation-manager is used.

Bug: 890385
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I86913ed64ac0c6982fae48b71f0561f071b4db30
Reviewed-on: https://chromium-review.googlesource.com/c/1252850
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605091}

ios/chrome/test/app/chrome_test_util.mm

@@ -6,6 +6,7 @@
 
 #include "base/logging.h"
 #include "base/mac/foundation_util.h"
+#import "base/test/ios/wait_util.h"
 #include "components/metrics/metrics_pref_names.h"
 #include "components/metrics/metrics_service.h"
 #import "ios/chrome/app/application_delegate/metrics_mediator.h"
@@ -27,10 +28,11 @@
 #import "ios/chrome/browser/ui/main/view_controller_swapping.h"
 #import "ios/chrome/browser/ui/ntp/new_tab_page_controller.h"
 #include "ios/chrome/browser/ui/util/ui_util.h"
-#include "ios/chrome/test/app/navigation_test_util.h"
 #import "ios/chrome/test/app/tab_test_util.h"
 #import "ios/web/public/navigation_manager.h"
+#include "ios/web/public/test/fakes/test_web_state_observer.h"
 #import "ios/web/public/test/native_controller_test_util.h"
+#import "ios/web/public/web_state/navigation_context.h"
 #import "third_party/breakpad/breakpad/src/client/ios/BreakpadController.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
@@ -239,10 +241,26 @@ void OpenChromeFromExternalApp(const GURL& url) {
 
 bool PurgeCachedWebViewPages() {
   web::WebState* web_state = chrome_test_util::GetCurrentWebState();
+  const GURL last_committed_url = web_state->GetLastCommittedURL();
+
   web_state->SetWebUsageEnabled(false);
   web_state->SetWebUsageEnabled(true);
+
+  auto observer = std::make_unique<web::TestWebStateObserver>(web_state);
+  web::TestWebStateObserver* observer_ptr = observer.get();
+
   web_state->GetNavigationManager()->LoadIfNecessary();
-  return chrome_test_util::WaitForPageToFinishLoading();
+
+  // The navigation triggered by LoadIfNecessary() may only start loading in the
+  // next run loop, if it is for a web URL. The most reliable way to detect that
+  // this navigation has finished is via the WebStateObserver.
+  return base::test::ios::WaitUntilConditionOrTimeout(
+      base::test::ios::kWaitForPageLoadTimeout, ^{
+        return observer_ptr->did_finish_navigation_info() &&
+               observer_ptr->did_finish_navigation_info()->context &&
+               observer_ptr->did_finish_navigation_info()->context->GetUrl() ==
+                   last_committed_url;
+      });
 }
 
 }  // namespace chrome_test_util