[Nav Experiment] Only create pending item for main frame decidePolicy.

|webView:decidePolicyForNavigationAction| is called on subresources
load as well. Pending item and context should not be updated in this
case, otherwise it causes the incorrect CL to be displayed.

Bug: 865893, 866379
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Icd721c8044b32cb5088631812ffe2de3c95c9c1b
Reviewed-on: https://chromium-review.googlesource.com/1152112Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578637}

ios/web/web_state/ui/crw_web_controller.mm

@@ -4205,6 +4205,7 @@ registerLoadRequestForURL:(const GURL&)requestURL
   [self updatePendingNavigationInfoFromNavigationAction:action];
 
   if (web::GetWebClient()->IsSlimNavigationManagerEnabled() &&
+      action.targetFrame.mainFrame &&
       action.navigationType == WKNavigationTypeBackForward) {
     // WKBackForwardList would have already been updated for back/forward
     // navigation. Create the pending item here to match.