gfx: Make conversions from Size to SizeF be explicit.

cc/base/math_util.h

 // Copyright 2012 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 CC_BASE_MATH_UTIL_H_
 #define CC_BASE_MATH_UTIL_H_
 
 #include <algorithm>
 #include <cmath>
 #include <vector>
@@ skipping 12 matching lines @@
 class Value;
 namespace trace_event {
 class TracedValue;
 }
 }  // namespace base
 
 namespace gfx {
 class QuadF;
 class Rect;
 class RectF;
+class SizeF;
 class Transform;
 class Vector2dF;
 class Vector2d;
 class Vector3dF;
 }
 
 namespace cc {
 
 struct HomogeneousCoordinate {
   HomogeneousCoordinate(SkMScalar x, SkMScalar y, SkMScalar z, SkMScalar w) {
@@ skipping 272 matching lines @@
   template <typename T>
   static T RoundDownInternal(T n, T mul) {
     return (n > 0) ? (n / mul) * mul : (n == 0) ? 0
                                                 : ((n - mul + 1) / mul) * mul;
   }
 };
 
 }  // namespace cc
 
 #endif  // CC_BASE_MATH_UTIL_H_