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..a18165c99a179341ec84029b5c79831fd3b26861 100644
--- a/Source/core/html/canvas/CanvasPathMethods.cpp
+++ b/Source/core/html/canvas/CanvasPathMethods.cpp
@@ -180,9 +180,14 @@ void canonicalizeAngle(float* startAngle, float* endAngle)
 {
     // Make 0 <= startAngle < 2*PI
     float newStartAngle = fmodf(*startAngle, twoPiFloat);
+
     if (newStartAngle < 0)
         newStartAngle += twoPiFloat;
 
+    // Compensate for numerical precision issues in fmodf (c.f. crbug.com/503422)

[Stephen White, 2015/06/26 15:38:21]

This is probably just loss of precision on the + above, where the
slightly-negative value added to twoPiFloat produces exactly twoPiFloat. As
discussed, let's not blame fmodf. :)

+    if (newStartAngle >= twoPiFloat)
+        newStartAngle -= twoPiFloat;
+
     float delta = newStartAngle - *startAngle;
     *startAngle = newStartAngle;
     *endAngle = *endAngle + delta;