Fixes an ARC migration error in TabSwitcherLocalSessionCell.

The migration tools inadvertently replaced autorelease with "= nil" in this
case.

Originally introducted in 9d30c08f.
Surrounding code was fixed in e5a9cd7d, but this
line was overlooked.

BUG=None

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I0872c29999934c04e330cf2e3f20b6d114d2657f
Reviewed-on: https://chromium-review.googlesource.com/749521
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513262}

ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_cell.mm

@@ -324,7 +324,7 @@ CGFloat tabSwitcherLocalSessionCellTopBarHeight() {
           initWithName:l10n_util::GetNSString(IDS_IOS_TAB_SWITCHER_CLOSE_TAB)
                 target:self
               selector:@selector(closeButtonPressed)];
-  [customActions addObject:customAction = nil];
+  [customActions addObject:customAction];
   return customActions;
 }