[iOS] Update shouldUseViewContentInset for smooth scrolling experiment.

This CL updates CRWWebViewProxy.shouldUseViewContentInset to be accurate
for the smooth scrolling experiment.

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I19036cc253ed52f99de5fc51949f96f8feabcd4c
Reviewed-on: https://chromium-review.googlesource.com/1176965Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586325}

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

@@ -22,6 +22,8 @@
 #error "This file requires ARC support."
 #endif
 
+using fullscreen::features::ViewportAdjustmentExperiment;
+
 namespace {
 // Returns whether fullscreen should be disabled for |web_state|'s SSL status.
 // This will return true if the visible NavigationItem's SSL has a broken
@@ -93,9 +95,11 @@ void FullscreenWebStateObserver::DidFinishNavigation(
   // - For normal pages, using |contentInset| breaks the layout of fixed-
   //   position DOM elements, so top padding must be accomplished by updating
   //   the WKWebView's frame.
+  ViewportAdjustmentExperiment viewport_experiment =
+      fullscreen::features::GetActiveViewportExperiment();
   bool force_content_inset =
-      fullscreen::features::GetActiveViewportExperiment() ==
-      fullscreen::features::ViewportAdjustmentExperiment::CONTENT_INSET;
+      viewport_experiment == ViewportAdjustmentExperiment::CONTENT_INSET ||
+      viewport_experiment == ViewportAdjustmentExperiment::SMOOTH_SCROLLING;
   web_state->GetWebViewProxy().shouldUseViewContentInset =
       force_content_inset ||
       web_state->GetContentsMimeType() == "application/pdf";