Add unittests for default font constructor.

This CL is adding a test to cover:
  Font::Font(...)
  PlatformFont::CreateDefault(...)

The default constructor was not covered by any unittests. This was found
while doing the migration from PlatformFontWin to PlatformFontSkia.

This CL is also restricting the use Font::platform_font(...) to windows.
This function will be removed as soon as the migration is completed.

R=robliao@chromium.org, asvitkine@chromium.org
CC=benck@google.com

Bug: 944227
Change-Id: Ib9a77c412966c3ebff81dfbf5340a762f69dc6e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642816
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666373}

ui/gfx/font.h

@@ -124,9 +124,11 @@ class GFX_EXPORT Font {
   NativeFont GetNativeFont() const;
 #endif
 
+#if defined(OS_WIN)
   // Raw access to the underlying platform font implementation. Can be
   // static_cast to a known implementation type if needed.
   PlatformFont* platform_font() const { return platform_font_.get(); }
+#endif
 
  private:
   // Wrapped platform font implementation.

ui/gfx/font_unittest.cc

@@ -13,7 +13,6 @@
 #include "ui/gfx/font_names_testing.h"
 
 #if defined(OS_WIN)
-#include "ui/gfx/platform_font_win.h"
 #include "ui/gfx/system_fonts_win.h"
 #endif
 
@@ -22,6 +21,19 @@ namespace {
 
 using FontTest = testing::Test;
 
+TEST_F(FontTest, DefaultFont) {
+  Font cf;
+  EXPECT_EQ(cf.GetStyle(), Font::NORMAL);
+  EXPECT_EQ(cf.GetWeight(), Font::Weight::NORMAL);
+  // Ensures that font metrics are generated. Some fonts backends do not provide
+  // some metrics (e.g. DWrite do not produce average character width).
+  EXPECT_GT(cf.GetFontSize(), 0);
+  EXPECT_GT(cf.GetHeight(), 0);
+  EXPECT_GT(cf.GetBaseline(), 0);
+  EXPECT_GT(cf.GetCapHeight(), 0);
+  EXPECT_GT(cf.GetExpectedTextWidth(1), 0);
+}
+
 TEST_F(FontTest, LoadArial) {
   Font cf(kTestFontName, 16);
 #if defined(OS_MACOSX) || defined(OS_IOS)