[ios] Do not update fullscreen viewport insets when flag is on

When FullscreenController is scoped to the Browser, fullscreen
viewport inset updates are handled downstream, so BVC no longer
has to respond to FullscreenUIElement callbacks by updating
the viewport insets.

Bug: 790886, 1090044
Change-Id: I52e08694b38d36ae6137b641b2b9be07a7bd4191
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2222292
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773782}

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -3501,7 +3501,9 @@ NSString* const kBrowserViewControllerSnackbarCategory =
 - (void)updateForFullscreenProgress:(CGFloat)progress {
   [self updateHeadersForFullscreenProgress:progress];
   [self updateFootersForFullscreenProgress:progress];
+  if (!fullscreen::features::ShouldScopeFullscreenControllerToBrowser()) {
     [self updateBrowserViewportForFullscreenProgress:progress];
+  }
 }
 
 - (void)updateForFullscreenEnabled:(BOOL)enabled {