Make ToolbarButton default constructible

Bug: 1108460
Change-Id: I5035011d8533c7f6de436c1384d110efa92636af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451524
Auto-Submit: Wei Li <weili@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814260}

chrome/browser/ui/views/toolbar/toolbar_button.h

@@ -44,7 +44,7 @@ class ToolbarButton : public views::LabelButton,
 
   // More convenient form of the ctor below, when |model| and |tab_strip_model|
   // are both nullptr.
-  explicit ToolbarButton(PressedCallback callback);
+  explicit ToolbarButton(PressedCallback callback = PressedCallback());
 
   // |tab_strip_model| must outlive this class.
   // |model| can be null if no menu is to be shown.

chrome/browser/ui/views/toolbar/toolbar_button_unittest.cc

@@ -96,14 +96,14 @@ class TestToolbarButton : public ToolbarButton {
 using ToolbarButtonViewsTest = ChromeViewsTestBase;
 
 TEST_F(ToolbarButtonViewsTest, DefaultLayoutInsets) {
-  ToolbarButton button{views::Button::PressedCallback()};
+  ToolbarButton button;
   gfx::Insets default_insets = ::GetLayoutInsets(TOOLBAR_BUTTON);
   EXPECT_EQ(default_insets, button.GetLayoutInsets());
   EXPECT_EQ(default_insets, button.GetInsets());
 }
 
 TEST_F(ToolbarButtonViewsTest, SetLayoutInsets) {
-  ToolbarButton button{views::Button::PressedCallback()};
+  ToolbarButton button;
   gfx::Insets new_insets(2, 3, 4, 5);
   button.SetLayoutInsets(new_insets);
   EXPECT_EQ(new_insets, button.GetLayoutInsets());