This simplifies transition functions to make it easier to reliably
elide comments in a later CL.

Before:
- transition functions are responsible for detecting special end tags.
After:
- the code to detect special end tags is done in one place.

We were relying on end tags being skipped which meant we were
not noticing comments inside script/style elements that contain no
substitutions.
This change means we will notice all such comments where necessary,
but stripTags will notice none since it does not need to.  This speeds
up stripTags.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5074041