Fixing bad angle computation in 2D canvas arc rendering

Source/core/html/canvas/CanvasPathMethods.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012, 2013 Intel Corporation. All rights reserved.
  * Copyright (C) 2012, 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 162 matching lines @@
 
 inline FloatPoint getPointOnEllipse(float radiusX, float radiusY, float theta)
 {
     return FloatPoint(radiusX * cosf(theta), radiusY * sinf(theta));
 }
 
 void canonicalizeAngle(float* startAngle, float* endAngle)
 {
     // Make 0 <= startAngle < 2*PI
     float newStartAngle = fmodf(*startAngle, twoPiFloat);
-    if (newStartAngle < 0)
+
+    if (newStartAngle < 0) {
         newStartAngle += twoPiFloat;
+        // Check for possible catastrophic cancellation in cases where
+        // newStartAngle was a tiny negative number (c.f. crbug.com/503422)
+        if (newStartAngle >= twoPiFloat)
+            newStartAngle -= twoPiFloat;
+    }
 
     float delta = newStartAngle - *startAngle;
     *startAngle = newStartAngle;
     *endAngle = *endAngle + delta;
 
     ASSERT(newStartAngle >= 0 && newStartAngle < twoPiFloat);
 }
 
 /*
  * degenerateEllipse() handles a degenerated ellipse using several lines.
@@ skipping 116 matching lines @@
         return;
 
     if (!width && !height) {
         m_path.moveTo(FloatPoint(x, y));
         return;
     }
 
     m_path.addRect(FloatRect(x, y, width, height));
 }
 }