Fixing bad angle computation in 2D canvas arc rendering

Source/core/html/canvas/CanvasPathMethods.cpp

Index: Source/core/html/canvas/CanvasPathMethods.cpp
diff --git a/Source/core/html/canvas/CanvasPathMethods.cpp b/Source/core/html/canvas/CanvasPathMethods.cpp
index 85a13a49e65878d060ad0f58e72d77facd684638..3d9ccd2ab2e584e04a50fc0260d2c36876737e73 100644
--- a/Source/core/html/canvas/CanvasPathMethods.cpp
+++ b/Source/core/html/canvas/CanvasPathMethods.cpp
@@ -180,8 +180,14 @@ 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;