[headless] Refactor headless devtools client API.

This patch splits types.{h,cc} and type_conversions.h into multiple
files on per-domain basis. This makes them more readable and reduces
compilation times.

Aliases are avalaible at the old locations for the transition period.

BUG=546953

Review-Url: https://codereview.chromium.org/2473073003
Cr-Commit-Position: refs/heads/master@{#430774}