Implement ui/gfx/geometry StructTraits unit tests

ui/gfx/geometry/mojo/geometry.mojom

 // 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.
 
 module gfx.mojom;
 
 struct Point {
   int32 x;
   int32 y;
 };
 
 struct PointF {
   float x;
   float y;
 };
 
 struct Size {
   int32 width;
   int32 height;
 };
 
 struct SizeF {
-  int32 width;
-  int32 height;
+  float width;
+  float height;
 };
 
 struct Rect {
   int32 x;
   int32 y;
   int32 width;
   int32 height;
 };
 
 struct RectF {
   float x;
   float y;
   float width;
   float height;
 };
 
 struct Insets {
   int32 top;
   int32 left;
   int32 bottom;
   int32 right;
 };
 
 struct InsetsF {
   float top;
   float left;
   float bottom;
   float right;
 };