experiment with zero-length round capped line segments

experiment with zero-length round capped line segments

If the endcap is not butt, draw the endcaps even when the line
has zero length.

If the dash length is zero, generate a zero length line segment.

Treat a move followed by a close as a move followed by a zero-length
line.

R=reed@google.com,schenney@google.com
BUG=422974

Review URL: https://codereview.chromium.org/1309753002