gfx: Make conversions from Size to SizeF be explicit.

ui/gfx/geometry/size_f.h

Index: ui/gfx/geometry/size_f.h
diff --git a/ui/gfx/geometry/size_f.h b/ui/gfx/geometry/size_f.h
index 66080f09c12d660c78bcab789e5e6caaeb38597e..2ee2f51973c635e96ad0236067d5ae9fedb88482 100644
--- a/ui/gfx/geometry/size_f.h
+++ b/ui/gfx/geometry/size_f.h
@@ -10,6 +10,7 @@
 #include <string>
 
 #include "base/compiler_specific.h"
+#include "ui/gfx/geometry/size.h"
 #include "ui/gfx/gfx_export.h"
 
 namespace gfx {
@@ -22,6 +23,10 @@ class GFX_EXPORT SizeF {
       : 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_; }