Add instructions for the Closure externs generator

Add instructions for the Closure externs generator

Generated externs instruct maintainers to reference this doc to
regenerate them, so add the expected instructions here.

R=dbeam@chromium.org
BUG=none

Review-Url: https://codereview.chromium.org/2852143002
Cr-Commit-Position: refs/heads/master@{#468487}
Committed: https://chromium.googlesource.com/chromium/src/+/c1e2d1ed8a403ce0e350aebadd12e0c38102e33d

[michaelpg, 2017-05-01 19:57:20]

PTAL.

[Dan Beam, 2017-05-01 20:38:38]

if only we had a presubmit check to verify that when an idl/json schema was
changed, an externs js was updated... hmmmm ;)

lgtm

[Devlin, 2017-05-01 20:42:45]

Kinda like this?

https://chromium.googlesource.com/chromium/src/+/de4c4327d2096b6d686b80a4cf48...

It mostly stalled out because we don't have separate externs for all our APIs. 
I think I added most of the ones that did have their own extern file to the
list.  Feel free to add more. :)

[michaelpg, 2017-05-01 23:29:29]

The CQ bit was checked by michaelpg@chromium.org

[commit-bot: I haz the power, 2017-05-01 23:30:11]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-01 23:41:02]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1493681369145580, "parent_rev":
"29fdc161a3c313e67f09e0ef891b747005a4ccae", "commit_rev":
"c1e2d1ed8a403ce0e350aebadd12e0c38102e33d"}

[commit-bot: I haz the power, 2017-05-01 23:41:14]

Description was changed from

==========
Add instructions for the Closure externs generator

Generated externs instruct maintainers to reference this doc to
regenerate them, so add the expected instructions here.

R=dbeam@chromium.org
BUG=none
==========

to

==========
Add instructions for the Closure externs generator

Generated externs instruct maintainers to reference this doc to
regenerate them, so add the expected instructions here.

R=dbeam@chromium.org
BUG=none

Review-Url: https://codereview.chromium.org/2852143002
Cr-Commit-Position: refs/heads/master@{#468487}
Committed:
https://chromium.googlesource.com/chromium/src/+/c1e2d1ed8a403ce0e350aebadd12...
==========

[commit-bot: I haz the power, 2017-05-01 23:41:16]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/c1e2d1ed8a403ce0e350aebadd12...