Move docs/es6_chromium.md to styleguide/web/es6.md instead

This document is far more a styleguide than something tracking
capabilities of a system.  There's many things that are functionally
available in Chromium/V8 but that we're choosing not to use yet for
various reasons (i.e. tooling support).

[email protected]
BUG=671426

Change-Id: I588fd338e89813200e934a00d7ac69c325d07968
Reviewed-on: https://chromium-review.googlesource.com/c/1257544
Reviewed-by: Dirk Pranke <[email protected]>
Reviewed-by: Demetrios Papadopoulos <[email protected]>
Commit-Queue: Dan Beam (no longer on Chrome) <[email protected]>
Cr-Commit-Position: refs/heads/master@{#596057}
3 files changed