Add support for new canvas ellipse method.

Source/core/platform/graphics/Path.cpp

 /*
  * Copyright (C) 2003, 2006 Apple Computer, Inc.  All rights reserved.
  *                     2006 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 287 matching lines @@
         sweepDegrees += s360;
 
     m_path.arcTo(oval, startDegrees, sweepDegrees, false);
 }
 
 void Path::addRect(const FloatRect& rect)
 {
     m_path.addRect(rect);
 }
 
+void Path::addEllipse(const FloatPoint& p, float radiusX, float radiusY, float rotation, float startAngle, float endAngle, bool anticlockwise)
+{
+    // Optimize the common case of an entire ellipse.
+    SkScalar twoPiScalar = WebCoreFloatToSkScalar(2 * piFloat);
+    SkScalar endAngleScalar = WebCoreFloatToSkScalar(endAngle);
+    if (!rotation && !startAngle && SkScalarNearlyEqual(twoPiScalar, SkScalarAbs(endAngleScalar))) {

[Stephen White, 2013/07/09 13:58:08]

If we're doing an approximate comparison for the end angle, shouldn't we also be
doing an approximate comparison for the start angle? Or exact for both?

[dshwang, 2013/07/09 14:42:05]

I made it with intention. Let me explain.

First of all, float a = 0 is just zero, not 0.0000000000001.

Sencond, I'm not sure that Math.PI * 2 in javascript equals 2 * piFloat in cpp.
Could I ensure they are exact in X86, X64 and ARM?

In addition, I can not expect how web developer use 2Pi. Smart developers will
use Math.PI * 2. However, I found that some code use something like 3.1415 * 2.
For example, fast/canvas/canvas-transforms-during-path.html

So imho, this code looks good to me. Could you feedback again? :)

+        FloatRect boundingRect(p - FloatSize(radiusX, radiusY), FloatSize(2 * radiusX, 2 * radiusY));
+        if (anticlockwise && SkScalarNearlyEqual(twoPiScalar, -endAngleScalar)) {
+            m_path.addOval(boundingRect, SkPath::kCCW_Direction);
+            return;
+        }
+        if (!anticlockwise && SkScalarNearlyEqual(twoPiScalar, endAngleScalar)) {
+            m_path.addOval(boundingRect);
+            return;
+        }
+    }
+
+    // Add an arc after the relevant transform.
+    AffineTransform ellipseTransform = AffineTransform::translation(p.x(), p.y()).rotate(rad2deg(rotation)).scale(radiusX, radiusY);
+    ASSERT(ellipseTransform.isInvertible());
+    AffineTransform inverseEllipseTransform = ellipseTransform.inverse();
+    transform(inverseEllipseTransform);
+    addArc(FloatPoint::zero(), 1 /* unit circle */, startAngle, endAngle, anticlockwise);
+    transform(ellipseTransform);
+}
+
 void Path::addEllipse(const FloatRect& rect)
 {
     m_path.addOval(rect);
 }
 
 void Path::addRoundedRect(const RoundedRect& r)
 {
     addRoundedRect(r.rect(), r.radii().topLeft(), r.radii().topRight(), r.radii().bottomLeft(), r.radii().bottomRight());
 }
 
@@ skipping 83 matching lines @@
 
     closeSubpath();
 }
 
 void Path::translate(const FloatSize& size)
 {
     m_path.offset(WebCoreFloatToSkScalar(size.width()), WebCoreFloatToSkScalar(size.height()));
 }
 
 }