Clean up the top level BUILD.gn file.

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}

[brettw, 2015-02-27 20:57:40]

lgtm

[Dirk Pranke, 2015-02-27 21:12:42]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-27 21:13:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/963093003/20001

[commit-bot: I haz the power, 2015-02-27 21:46:53]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-27 21:48:11]

Patchset 2 (id:??) landed as
https://crrev.com/18e5743bbe39f30b75abad4c5c7f0efab288ed9a
Cr-Commit-Position: refs/heads/master@{#318523}

[eroman, 2015-02-27 22:03:54]

eroman@chromium.org changed reviewers:
+ eroman@chromium.org

[eroman, 2015-02-27 22:03:54]

Looks like this caused a break on mac:

http://build.chromium.org/p/chromium.mac/builders/Mac%20GN%20%28dbg%29/builds...

[eroman, 2015-02-27 22:05:07]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/962383002/ by eroman@chromium.org.

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

[Dirk Pranke, 2015-02-27 22:05:53]

On 2015/02/27 22:05:07, eroman wrote:
> A revert of this CL (patchset #2 id:20001) has been created in
> https://codereview.chromium.org/962383002/ by mailto:eroman@chromium.org.
> 
> The reason for reverting is: 
> ERROR at //BUILD.gn:246:7: Item not found
>       "//third_party/usrsctp",  # TODO(GYP)
>       ^----------------------
> You were trying to remove "//third_party/usrsctp".

Hrm. Guess I missed that part. Thanks for catching it!