Implement "auto -> angle" animation on <marker> orient attribute.

Before this CL, "angle -> auto" animation was implemented but "auto -> angle"
animation was not implemented. This CL implements the missing piece as
a discrete animation as specified in the spec.

BUG=425848

Review URL: https://codereview.chromium.org/672853002

git-svn-id: svn://svn.chromium.org/blink/trunk@184260 bbb929c8-8fbe-4397-9dbb-9b2b20218538