Introduce ViewsTestBase::AllocateTestWidget().

This provides a customization point for subclasses to override, to
change the Widget subclass that's instantiated by CreateTestWidget().
This will be necessary to remove DefaultThemeProvider, because
ChromeViewsTestBase will need to provide widgets with a non-null
ThemeProvider at that point, and subclassing Widget is the only way to
do so.

Bug: none
Change-Id: I9057301e2a86449832e036ce943d595e56954e74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143518
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757880}

ui/views/test/views_test_base.cc

@@ -132,7 +132,7 @@ Widget::InitParams ViewsTestBase::CreateParams(Widget::InitParams::Type type) {
 std::unique_ptr<Widget> ViewsTestBase::CreateTestWidget(
     Widget::InitParams::Type type) {
   Widget::InitParams params = CreateParamsForTestWidget(type);
-  auto widget = std::make_unique<Widget>();
+  std::unique_ptr<Widget> widget = AllocateTestWidget();
   widget->Init(std::move(params));
   return widget;
 }
@@ -183,6 +183,10 @@ NativeWidget* ViewsTestBase::CreateNativeWidgetForTest(
 #endif
 }
 
+std::unique_ptr<Widget> ViewsTestBase::AllocateTestWidget() {
+  return std::make_unique<Widget>();
+}
+
 Widget::InitParams ViewsTestBase::CreateParamsForTestWidget(
     Widget::InitParams::Type type) {
   Widget::InitParams params = CreateParams(type);

ui/views/test/views_test_base.h

@@ -121,6 +121,11 @@ class ViewsTestBase : public PlatformTest {
       const Widget::InitParams& init_params,
       internal::NativeWidgetDelegate* delegate);
 
+  // Instantiates a Widget for CreateTestWidget(), but does no other
+  // initialization.  Overriding this allows subclasses to customize the Widget
+  // subclass returned from CreateTestWidget().
+  virtual std::unique_ptr<Widget> AllocateTestWidget();
+
   Widget::InitParams CreateParamsForTestWidget(
       Widget::InitParams::Type type =
           Widget::InitParams::TYPE_WINDOW_FRAMELESS);