Update GN documentation.

Update GN documentation.

This CL updates the main GN page to reflect that the migration is
basically done, and also adds documentation for the magic
top-level "//:default" target (which we don't actually use in Chromium).

R=brettw@chromium.org, kjellander@chromium.org

Committed: https://crrev.com/e5e5a9dd1ca6b2d16db5ca08031ccade0654504f
Cr-Commit-Position: refs/heads/master@{#423973}

[Dirk Pranke, 2016-10-07 20:30:45]

Patchset #1 (id:1) has been deleted

[brettw, 2016-10-07 21:34:00]

lgtm

[Dirk Pranke, 2016-10-07 21:34:32]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2016-10-07 21:35:15]

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, 2016-10-07 21:45:10]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2016-10-07 21:47:01]

Description was changed from

==========
Update GN documentation.

This CL updates the main GN page to reflect that the migration is
basically done, and also adds documentation for the magic
top-level "//:default" target (which we don't actually use in Chromium).

R=brettw@chromium.org, kjellander@chromium.org
==========

to

==========
Update GN documentation.

This CL updates the main GN page to reflect that the migration is
basically done, and also adds documentation for the magic
top-level "//:default" target (which we don't actually use in Chromium).

R=brettw@chromium.org, kjellander@chromium.org

Committed: https://crrev.com/e5e5a9dd1ca6b2d16db5ca08031ccade0654504f
Cr-Commit-Position: refs/heads/master@{#423973}
==========

[commit-bot: I haz the power, 2016-10-07 21:47:02]

Patchset 1 (id:??) landed as
https://crrev.com/e5e5a9dd1ca6b2d16db5ca08031ccade0654504f
Cr-Commit-Position: refs/heads/master@{#423973}

[kjellander_chromium, 2016-10-09 03:29:22]

lgtm!