Instead of making them an extra option that gets passed in and compiled
at the end of the natives file for a given run of js2c, we now make them a
separate run of js2c with a separate natives file output.

This natives file output is then compiled in the bootstrapper. It is not part
of the snapshot (yet), but instead is treated similar to the experimental
natives, just without any of the complexity that comes from tieing the
behavior to flags. We also don't need counterparts to
InitializeExperimentalGlobal and InstallExperimentalNativeFunctions (yet?).

This fixes the issue with https://codereview.chromium.org/1129743003 by making
the dummy file that is generated for snapshots with no extras (or no experimental
features) nonempty.

R=yangguo@chromium.org, jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1130993003

Cr-Commit-Position: refs/heads/master@{#28311}