Make Checkbox Default Constructible

BUG=1108460

Change-Id: Ib980a52cb155a32eeaa2412dfb70e496c1bf24c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315088
Commit-Queue: Robert Liao <robliao@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Auto-Submit: Robert Liao <robliao@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791397}

ui/views/controls/button/checkbox.h

@@ -28,7 +28,7 @@ class VIEWS_EXPORT Checkbox : public LabelButton {
   METADATA_HEADER(Checkbox);
 
   // |force_md| forces MD even when --secondary-ui-md flag is not set.
-  explicit Checkbox(const base::string16& label,
+  explicit Checkbox(const base::string16& label = base::string16(),
                     ButtonListener* listener = nullptr);
   ~Checkbox() override;
 

ui/views/controls/button/checkbox_unittest.cc

@@ -32,8 +32,7 @@ class CheckboxTest : public ViewsTestBase {
     widget_->Init(std::move(params));
     widget_->Show();
 
-    checkbox_ =
-        widget_->SetContentsView(std::make_unique<Checkbox>(base::string16()));
+    checkbox_ = widget_->SetContentsView(std::make_unique<Checkbox>());
   }
 
   void TearDown() override {