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

ui/gfx/geometry/vector2d_f.h

Index: ui/gfx/geometry/vector2d_f.h
diff --git a/ui/gfx/geometry/vector2d_f.h b/ui/gfx/geometry/vector2d_f.h
index 593e7b82bdf81d6999868f7c08804a648e064ce4..3e1152a56471832510c3b8af64509a7b09a1d923 100644
--- a/ui/gfx/geometry/vector2d_f.h
+++ b/ui/gfx/geometry/vector2d_f.h
@@ -19,13 +19,13 @@ namespace gfx {
 
 class GFX_EXPORT Vector2dF {
  public:
-  Vector2dF() : x_(0), y_(0) {}
-  Vector2dF(float x, float y) : x_(x), y_(y) {}
+  constexpr Vector2dF() : x_(0), y_(0) {}
+  constexpr Vector2dF(float x, float y) : x_(x), y_(y) {}
 
-  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; }
 
   // True if both components of the vector are 0.