Use vertex attributes for dash effect in gpu

Use vertex attributes for dash effect in gpu

This will allow us to batch dashed lines together when drawing. Also, this removes the need for
a coord transform matrix in the shader, thus we save the cost of uploading a new matrix uniform
everytime we do a simple transform to the dashed line we are drawing.

BUG=skia:
R=bsalomon@google.com

Author: egdaniel@google.com

Review URL: https://codereview.chromium.org/326103002