Change Size::IsEmpty() to be consistent with Rect::IsEmpty()

base/gfx/size.h

Index: base/gfx/size.h
===================================================================
--- base/gfx/size.h	(revision 14559)
+++ base/gfx/size.h	(working copy)
@@ -23,7 +23,7 @@
 class Size {
  public:
   Size() : width_(0), height_(0) {}
-  Size(int width, int height) : width_(width), height_(height) {}
+  Size(int width, int height);
 
   ~Size() {}
 
@@ -31,17 +31,17 @@
   int height() const { return height_; }
 
   void SetSize(int width, int height) {
-    width_ = width;
-    height_ = height;
+    set_width(width);
+    set_height(height);
   }
 
   void Enlarge(int width, int height) {
-    width_ += width;
-    height_ += height;
+    set_width(width_ + width);
+    set_height(height_ + height);
   }
 
-  void set_width(int width) { width_ = width; }
-  void set_height(int height) { height_ = height; }
+  void set_width(int width);
+  void set_height(int height);
 
   bool operator==(const Size& s) const {
     return width_ == s.width_ && height_ == s.height_;
@@ -52,7 +52,8 @@
   }
 
   bool IsEmpty() const {
-    return !width_ && !height_;
+    // Size doesn't allow negative dimensions, so testing for 0 is enough.
+    return (width_ == 0) || (height_ == 0);
   }
 
 #if defined(OS_WIN)