Removed shadowPseudoId and replaced setPseudo with setPart.

Still need to support custom pseudo elements for the followings:
- ::-webkit-XXX (used by forms and media)
- ::cue (used by media)

This patch does:
- replace setPseudo with setPart.
- remove setPseudo.
- change shadowPseudoId. now the method is something like "getPartOrPseudo".
- modify SelectorChecker to use pseudo() for CustomPseudo, and use shadowPseudoId for WebKitCustomPseudo.
- change ::cue's pseudoType from CustomPseudo to WebKitCustomPseudo.

- rebaseline affected layout tests, e.g. *[pseudo=XXX] is replaced with *[part=XXX].

BUG=254020
R=dglazkov@chromium.org, morrita@chromium.org

Review URL: https://codereview.chromium.org/20887003

git-svn-id: svn://svn.chromium.org/blink/trunk@155834 bbb929c8-8fbe-4397-9dbb-9b2b20218538