[ios] Remove more unnecessary references to Tab in BVC

Two private methods in BVC take in a WebState as an argument.
They call the TabHelper to get the corresponding Tab, then never use
the Tab reference.
This CL removes these unnecessary references to Tab.

Change-Id: Ie5be3d4600fafe9bd4777ece6717eb67525e8c30
Reviewed-on: https://chromium-review.googlesource.com/c/1344835Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610206}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -2993,8 +2993,6 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint
 
 - (BOOL)canTakeSnapshotForWebState:(web::WebState*)webState {
   DCHECK(webState);
-  Tab* tab = LegacyTabHelper::GetTabForWebState(webState);
-  DCHECK([self.tabModel indexOfTab:tab] != NSNotFound);
   PagePlaceholderTabHelper* pagePlaceholderTabHelper =
       PagePlaceholderTabHelper::FromWebState(webState);
   return !pagePlaceholderTabHelper->displaying_placeholder() &&
@@ -3003,9 +3001,6 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint
 
 - (UIEdgeInsets)snapshotEdgeInsetsForWebState:(web::WebState*)webState {
   DCHECK(webState);
-  Tab* tab = LegacyTabHelper::GetTabForWebState(webState);
-  DCHECK([self.tabModel indexOfTab:tab] != NSNotFound);
-
   // The NTP's snapshot should be inset |headerHeight| from the top to remove
   // the fake NTP toolbar from the snapshot.
   NewTabPageTabHelper* NTPHelper = NewTabPageTabHelper::FromWebState(webState);