Sync for validator/cpp/{engine,htmlparser}

[antiphoton]

Sync for validator/cpp/htmlparser
Sync for validator/cpp/engine

This PR syncs cl/378533243 for the two folders in one time, because neither can pass circle ci test by itself.

[amp-owners-bot]

Hey @ampproject/wg-caching! These files were changed:

validator/cpp/engine/parse-layout.cc
validator/cpp/engine/parse-layout.h
validator/cpp/engine/validator-internal.cc
validator/cpp/htmlparser/node.cc
validator/cpp/htmlparser/node.h
validator/cpp/htmlparser/parser.cc
validator/cpp/htmlparser/parser_test.cc
validator/cpp/htmlparser/token.h
validator/cpp/htmlparser/tokenizer.cc
validator/cpp/htmlparser/tokenizer.h
validator/cpp/htmlparser/tokenizer_test.cc