gfx: Make conversions from Size to SizeF be explicit.

ui/gfx/geometry/size_f.h

 // Copyright (c) 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 UI_GFX_GEOMETRY_SIZE_F_H_
 #define UI_GFX_GEOMETRY_SIZE_F_H_
 
 #include <cmath>
 #include <iosfwd>
 #include <string>
 
 #include "base/compiler_specific.h"
+#include "ui/gfx/geometry/size.h"
 #include "ui/gfx/gfx_export.h"
 
 namespace gfx {
 
 // A floating version of gfx::Size.
 class GFX_EXPORT SizeF {
  public:
   SizeF() : width_(0.f), height_(0.f) {}
   SizeF(float width, float height)
       : width_(fmaxf(0, width)), height_(fmaxf(0, height)) {}
   ~SizeF() {}
 
+  explicit SizeF(const Size& size)
+      : SizeF(static_cast<float>(size.width()),
+              static_cast<float>(size.height())) {}
+
   float width() const { return width_; }
   float height() const { return height_; }
 
   void set_width(float width) { width_ = fmaxf(0, width); }
   void set_height(float height) { height_ = fmaxf(0, height); }
 
   float GetArea() const;
 
   void SetSize(float width, float height) {
     set_width(width);
@@ skipping 37 matching lines @@
 }
 
 // This is declared here for use in gtest-based unit tests but is defined in
 // the gfx_test_support target. Depend on that to use this in your unit test.
 // This should not be used in production code - call ToString() instead.
 void PrintTo(const SizeF& size, ::std::ostream* os);
 
 }  // namespace gfx
 
 #endif  // UI_GFX_GEOMETRY_SIZE_F_H_