Revert of Clean up the top level BUILD.gn file.

Revert of Clean up the top level BUILD.gn file. (patchset #2 id:20001 of https://codereview.chromium.org/963093003/)

Reason for revert:

ERROR at //BUILD.gn:246:7: Item not found
      "//third_party/usrsctp",  # TODO(GYP)
      ^----------------------
You were trying to remove "//third_party/usrsctp"

Original issue's description:
> Clean up the top level BUILD.gn file.
>
> There was a bunch of todo-related stuff that could
> be cleaned up, and this change also removes the 'gyp_all'
> target which was mostly just duplicating stuff in all.gyp.
>
> This CL should have no actual affect on the builds, apart
> from deleting the one target, but is also a prelude to
> subsequent CLs for this bug.
>
> R=brettw@chromium.org
> BUG=461019
>
> Committed: https://crrev.com/18e5743bbe39f30b75abad4c5c7f0efab288ed9a
> Cr-Commit-Position: refs/heads/master@{#318523}

TBR=brettw@chromium.org,dpranke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=461019
Committed: https://crrev.com/130761b4c49b06f4b4f075df841e473cce2af208
Cr-Commit-Position: refs/heads/master@{#318527}

[eroman, 2015-02-27 22:05:08]

Created Revert of Clean up the top level BUILD.gn file.

[Dirk Pranke, 2015-02-27 22:06:51]

On 2015/02/27 22:05:08, eroman wrote:
> Created Revert of Clean up the top level BUILD.gn file.

lgtm, thanks!

[commit-bot: I haz the power, 2015-02-27 22:07:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/962383002/1

[commit-bot: I haz the power, 2015-02-27 22:08:15]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-27 22:09:11]

Patchset 1 (id:??) landed as
https://crrev.com/130761b4c49b06f4b4f075df841e473cce2af208
Cr-Commit-Position: refs/heads/master@{#318527}