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

ui/gfx/render_text_harfbuzz.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_RENDER_TEXT_HARFBUZZ_H_
 #define UI_GFX_RENDER_TEXT_HARFBUZZ_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 14 matching lines @@
 }
 
 namespace gfx {
 
 class Range;
 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
   // |pos|.
   size_t CharToGlyph(size_t pos) const;
 
   // Returns the corresponding glyph range of the given character range.
   // |range| is in text-space (0 corresponds to |GetDisplayText()[0]|). Returned
   // value is in run-space (0 corresponds to the first glyph in the run).
   Range CharRangeToGlyphRange(const Range& range) const;
@@ skipping 228 matching lines @@
 
   // Fixed width of glyphs. This should only be set in test environments.
   float glyph_width_for_test_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderTextHarfBuzz);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_RENDER_TEXT_HARFBUZZ_H_