Skia draws round-caps line segments as a concave shape so skia GPU backend will not use convex path renderer, which is faster. 

I tried to make the shape convex by changing the control points of the line segments a little. The performance is much better. The score of MotionMark Basic Canvas Suite - "Canvas line segments, round caps" subcase will nearly triple from my experiment.

The benchmark can be found here: http://browserbench.org/MotionMark/developer.html

My change is in this CL: https://codereview.chromium.org/2691843003. It's hacky and I think a better way is needed.