[iOS] Updates custom separator thickness

This CL updates the custom separator thickness to match the 'default'
separator.


Bug: 1106669
Change-Id: I49f8e0c1df6169826e27dfff0685ea9395922a05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352734
Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798090}

ios/chrome/browser/ui/table_view/cells/table_view_cell.mm

@@ -14,7 +14,7 @@
 #endif
 
 namespace {
-const CGFloat kTableViewCustomSeparatorHeight = 0.75;
+const CGFloat kTableViewCustomSeparatorHeight = 0.5;
 }  // namespace
 
 @interface TableViewCell ()