RenderText: Don't default-construct gfx::Fonts unnecessarily

ui/gfx/render_text_harfbuzz.h

Index: ui/gfx/render_text_harfbuzz.h
diff --git a/ui/gfx/render_text_harfbuzz.h b/ui/gfx/render_text_harfbuzz.h
index 38ee2b4f3c31465d04b1e69784d0d5e270af5163..f164dc1d42053139d1963b9c78fc0b4cf5164680 100644
--- a/ui/gfx/render_text_harfbuzz.h
+++ b/ui/gfx/render_text_harfbuzz.h
@@ -32,7 +32,9 @@ class RangeF;
 namespace internal {
 
 struct GFX_EXPORT TextRunHarfBuzz {
-  TextRunHarfBuzz();
+  // Construct the run with |template_font| since determining the details of a
+  // default-constructed gfx::Font is expensive, but it will always be replaced.
+  explicit TextRunHarfBuzz(const gfx::Font& template_font);
   ~TextRunHarfBuzz();
 
   // Returns the index of the first glyph that corresponds to the character at