Make various gfx classes more amenable to use as compile-time constants.

ui/gfx/geometry/vector3d_f.h

Index: ui/gfx/geometry/vector3d_f.h
diff --git a/ui/gfx/geometry/vector3d_f.h b/ui/gfx/geometry/vector3d_f.h
index 4e0eaa412492205b14e8ddb63a1d60c9cee4c15b..f6c23044d5a41b571e2d9329a27db8f4a12ef11e 100644
--- a/ui/gfx/geometry/vector3d_f.h
+++ b/ui/gfx/geometry/vector3d_f.h
@@ -20,18 +20,19 @@ namespace gfx {
 
 class GFX_EXPORT Vector3dF {
  public:
-  Vector3dF();
-  Vector3dF(float x, float y, float z);
+  constexpr Vector3dF() : x_(0), y_(0), z_(0) {}
+  constexpr Vector3dF(float x, float y, float z) : x_(x), y_(y), z_(z) {}
 
-  explicit Vector3dF(const Vector2dF& other);
+  constexpr explicit Vector3dF(const Vector2dF& other)
+      : x_(other.x()), y_(other.y()), z_(0) {}
 
-  float x() const { return x_; }
+  constexpr float x() const { return x_; }
   void set_x(float x) { x_ = x; }
 
-  float y() const { return y_; }
+  constexpr float y() const { return y_; }
   void set_y(float y) { y_ = y; }
 
-  float z() const { return z_; }
+  constexpr float z() const { return z_; }
   void set_z(float z) { z_ = z; }
 
   // True if all components of the vector are 0.