[iOS] Show toolbar for load events when UI refresh flag is enabled.

In the legacy UI, fullscren was disabled when the WKWebView was loading
so that the URL of the page is always visible.  After the UI refresh,
the URL is always visible, so fullscreen does not need to be disabled.
However, the toolbar should still be shown (but not disabled) upon load
events to ensure that navigation to AMP pages allows access to the
navigation controls on these pages.

Bug: 837254
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Icc9e54cd6972b481c9041fb547f4ab5e8ba40b81
Reviewed-on: https://chromium-review.googlesource.com/1048635
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556908}

ios/chrome/browser/ui/fullscreen/fullscreen_web_state_observer.mm

@@ -129,12 +129,12 @@ void FullscreenWebStateObserver::SetDisableFullscreenForSSL(bool disable) {
 }
 
 void FullscreenWebStateObserver::SetIsLoading(bool loading) {
-  if (IsUIRefreshPhase1Enabled())
-    return;
-
-  if (!!loading_disabler_.get() == loading)
-    return;
-  loading_disabler_ =
-      loading ? std::make_unique<ScopedFullscreenDisabler>(controller_)
-              : nullptr;
+  if (IsUIRefreshPhase1Enabled()) {
+    if (loading)
+      controller_->ResetModel();
+  } else {
+    loading_disabler_ =
+        loading ? std::make_unique<ScopedFullscreenDisabler>(controller_)
+                : nullptr;
+  }
 }