[ios] Fix crash in tab switcher

Crash is caused by the previousPanelIndex being out-of-bounds into the
panels array. This CL ensures that previousPanelIndex is kept up-to-date
when any panels are removed so that it is not out-of-bounds.

Bug: 805530
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id193a85585510c50e9bb92680f0782fee7e34d1a
Reviewed-on: https://chromium-review.googlesource.com/916943Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536745}

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

@@ -110,6 +110,17 @@ const CGFloat kNewTabButtonWidth = 48;
   DCHECK_EQ([[_panels objectAtIndex:index] superview], _scrollView);
   [[_panels objectAtIndex:index] removeFromSuperview];
   [_panels removeObjectAtIndex:index];
+
+  if (_previousPanelIndex > -1) {
+    NSUInteger previousPanelIndexUnsigned =
+        static_cast<NSUInteger>(_previousPanelIndex);
+    if (index < previousPanelIndexUnsigned)
+      _previousPanelIndex--;
+    else if (index == previousPanelIndexUnsigned) {
+      [self panelWasHiddenAtIndex:_previousPanelIndex];
+      _previousPanelIndex = -1;
+    }
+  }
   if (update)
     [self updateScrollViewContent];
 }
@@ -336,7 +347,9 @@ const CGFloat kNewTabButtonWidth = 48;
   _previousPanelIndex = panelIndex;
 }
 
-- (void)panelWasHiddenAtIndex:(NSInteger)index {
+- (void)panelWasHiddenAtIndex:(NSUInteger)index {
+  if (index >= [_panels count])
+    return;
   id panel = [_panels objectAtIndex:index];
   if ([panel respondsToSelector:@selector(wasHidden)])
     [panel wasHidden];