[Nav Experiment] RestoreSession after webUsageEnabled is true.

WKBasedNavigationManager needs to create a web view to restore session.
The current order causes an CHECK in CRWWebController that prevents
the creation of new web view when |_webUsageEnabled| is false.

This fixes failure in NavigationTestCase and crashes observed in the
wild.

Bug: 852847
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ia6a7800ca6ec985a2080d0e0acc3a743c8c62c5f
Reviewed-on: https://chromium-review.googlesource.com/1108575Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569259}

ios/web/web_state/web_state_impl.mm

@@ -568,6 +568,11 @@ bool WebStateImpl::IsWebUsageEnabled() const {
 }
 
 void WebStateImpl::SetWebUsageEnabled(bool enabled) {
+  // This must be called before RestoreSessionStorage() because the latter
+  // creates a web view under WKBasedNavigationManager, and expects
+  // _webUsageEnabled to be true.
+  [web_controller_ setWebUsageEnabled:enabled];
+
   // SetWebUsageEnabled(false) will cause the WKWebView to be removed and this
   // is the only way to clear browser data. Cache the session history in this
   // WebState so that when web usage is re-enabled, history can be restored into
@@ -583,8 +588,6 @@ void WebStateImpl::SetWebUsageEnabled(bool enabled) {
       cached_session_storage_ = BuildSessionStorage();
     }
   }
-
-  [web_controller_ setWebUsageEnabled:enabled];
 }
 
 bool WebStateImpl::ShouldSuppressDialogs() const {