Re-land the 'gn_migration.gyp' patch.

Re-land the 'gn_migration.gyp' patch.

This patch re-lands the original change plus the fixes I
had to introduce the 'gn_all' and 'gyp_remaining' targets
in the GYP build. This patch is thus reverting
378e940e (#318636).

The code was actually correct at #318636 but was failing
on the iOS bots due to problems w/ their recipes that have
since been fixed.

TBR=brettw@chromium.org
BUG=461019
Committed: https://crrev.com/5b90c6fdbd3097080752fcf15f2f0e5ae4d3aec3
Cr-Commit-Position: refs/heads/master@{#318960}

[Dirk Pranke, 2015-03-03 21:09:57]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-03-03 21:10:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/977703003/1

[commit-bot: I haz the power, 2015-03-03 21:16:16]

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

[commit-bot: I haz the power, 2015-03-03 21:16:17]

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...)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)

[Dirk Pranke, 2015-03-03 21:19:59]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-03-03 21:21:00]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/977703003/20001

[commit-bot: I haz the power, 2015-03-03 23:20:38]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-03 23:23:22]

Patchset 2 (id:??) landed as
https://crrev.com/5b90c6fdbd3097080752fcf15f2f0e5ae4d3aec3
Cr-Commit-Position: refs/heads/master@{#318960}