Fix checkbox focus ring

During refactoring in r703138 I forgot to install the new highlight-path
generator.

Bug: chromium:1013983
Change-Id: If39953bff877bf69066d954d1e7c04b042289636
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860152Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705709}

ui/views/controls/button/checkbox.cc

@@ -60,6 +60,8 @@ Checkbox::Checkbox(const base::string16& label, ButtonListener* listener)
   // Checkboxes always have a focus ring, even when the platform otherwise
   // doesn't generally use them for buttons.
   SetInstallFocusRingOnFocus(true);
+  focus_ring()->SetPathGenerator(
+      std::make_unique<FocusRingHighlightPathGenerator>());
 }
 
 Checkbox::~Checkbox() = default;