Re-land "Clean up the top level BUILD.gn file."

Re-land "Clean up the top level BUILD.gn file."

This re-lands #318523 with a fix for the mac build.

TBR=brettw@chromium.org
BUG=461019
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Committed: https://crrev.com/d62d851ee66c30eb639d9309b7e17631bbbfd354
Cr-Commit-Position: refs/heads/master@{#318638}

[Dirk Pranke, 2015-02-27 22:13:24]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-27 22:14:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967683003/20001

[commit-bot: I haz the power, 2015-02-27 22:38:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-27 22:39:00]

Try jobs failed on following builders:
  win8_chromium_gn_dbg on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_gn_...)

[Dirk Pranke, 2015-02-28 17:18:11]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-02-28 17:18:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967683003/40001

[commit-bot: I haz the power, 2015-02-28 17:23:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-02-28 17:23:30]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[Dirk Pranke, 2015-03-01 20:53:10]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-03-01 20:53:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967683003/60001

[commit-bot: I haz the power, 2015-03-01 20:57:43]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-03-01 20:57:44]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[Dirk Pranke, 2015-03-02 02:17:18]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-03-02 02:17:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/967683003/60001

[commit-bot: I haz the power, 2015-03-02 03:06:10]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-03-02 03:07:31]

Patchset 4 (id:??) landed as
https://crrev.com/d62d851ee66c30eb639d9309b7e17631bbbfd354
Cr-Commit-Position: refs/heads/master@{#318638}