Call WebState::WasHidden on Tab removal.

The primary purpose of this CL is to fix a bug, when
Download Manager UI persists after closing and reopening a tab.

There is an alternative fix which is to dismiss Download Manager
UI in DownloadManagerTabHelper::WebStateWasDestroyed, but calling
WebState::WasHidden for closed WebState sounds like the right
approach in general.

TBR=eugenebut@chromium.org

(cherry picked from commit 6ebb327)

Bug: 919974
Change-Id: I204514995d616297b0745763dcd5acc21afef678
Reviewed-on: https://chromium-review.googlesource.com/c/1433497
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#626608}
Reviewed-on: https://chromium-review.googlesource.com/c/1444140
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#56}
Cr-Branched-From: e510299-refs/heads/master@{#625896}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -4529,6 +4529,8 @@ - (void)tabModel:(TabModel*)model
 - (void)tabModel:(TabModel*)model
     didRemoveTab:(Tab*)tab
          atIndex:(NSUInteger)index {
+  tab.webState->WasHidden();
+
   [self uninstallDelegatesForTab:tab];
 
   // Cancel dialogs for |tab|'s WebState.