Document OS_CHROMEOS in build_config.h

Document OS_CHROMEOS in build_config.h

build_config.h *looks* like a list of all of the OS_* macros, but it is
missing OS_CHROMEOS. This can lead to confusion. Documenting OS_CHROMEOS
here makes build_config.h the source of truth that developers already
assume that it is.

Committed: https://crrev.com/8ad45afb36ab22013743ec48c7d1b34fc3bdd810
Cr-Commit-Position: refs/heads/master@{#436217}

[brucedawson, 2016-07-06 19:56:37]

brucedawson@chromium.org changed reviewers:
+ agrieve@chromium.org

[brucedawson, 2016-07-06 19:56:38]

Adding a comment to save future me from confusion. PTAL.

[agrieve, 2016-12-01 03:27:02]

On 2016/07/06 19:56:38, brucedawson wrote:
> Adding a comment to save future me from confusion. PTAL.

lgtm (missed this :P)

[brucedawson, 2016-12-05 01:11:19]

The CQ bit was checked by brucedawson@chromium.org

[commit-bot: I haz the power, 2016-12-05 01:11:33]

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, 2016-12-05 01:48:56]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1480900279315070,
"parent_rev": "0b78b3a46d6ca186d51af946ca44fd7ca95ce3dd", "commit_rev":
"c58b7b012d7e50ba110f4936fbae3a985db5a9d9"}

[commit-bot: I haz the power, 2016-12-05 01:49:22]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-12-05 01:52:23]

Description was changed from

==========
Document OS_CHROMEOS in build_config.h

build_config.h *looks* like a list of all of the OS_* macros, but it is
missing OS_CHROMEOS. This can lead to confusion. Documenting OS_CHROMEOS
here makes build_config.h the source of truth that developers already
assume that it is.
==========

to

==========
Document OS_CHROMEOS in build_config.h

build_config.h *looks* like a list of all of the OS_* macros, but it is
missing OS_CHROMEOS. This can lead to confusion. Documenting OS_CHROMEOS
here makes build_config.h the source of truth that developers already
assume that it is.

Committed: https://crrev.com/8ad45afb36ab22013743ec48c7d1b34fc3bdd810
Cr-Commit-Position: refs/heads/master@{#436217}
==========

[commit-bot: I haz the power, 2016-12-05 01:52:24]

Patchset 2 (id:??) landed as
https://crrev.com/8ad45afb36ab22013743ec48c7d1b34fc3bdd810
Cr-Commit-Position: refs/heads/master@{#436217}