Cleanup GN BUILD files, gni's and imports.

Cleanup GN BUILD files, gni's and imports.

Remove unused mojo/public/mojo_sdk.gni.
(we use third_party/mojo's version instead)

Update, use mojo/public/ mojo.gni and mojo_application.gni
(to define Chromium's mojo_root, mojo_native_application)
(we'll need a custom mojo_native_application for 498020)

Remove unnecessary imports of related gni files.

BUG=498020
TEST=Build still works as expected; no regressions.
R=sky@chromium.org
TBR=jochen@chromium.org,xhwang@chromium.org

Committed: https://crrev.com/4ece34be265cc6205eea813b6805e1659c3131ca
Cr-Commit-Position: refs/heads/master@{#333600}

[msw, 2015-06-09 20:21:24]

msw@chromium.org changed reviewers:
+ sky@chromium.org

[msw, 2015-06-09 20:21:39]

Hey Scott, please take a look; thanks!

[sky, 2015-06-09 20:55:49]

LGTM

[msw, 2015-06-09 20:57:02]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2015-06-09 20:57:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1175693002/40001

[commit-bot: I haz the power, 2015-06-09 21:10:12]

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

[commit-bot: I haz the power, 2015-06-09 21:10:13]

Try jobs failed on following builders:
  win8_chromium_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win8_chromium_ng/...)

[msw, 2015-06-09 21:52:24]

msw@chromium.org changed reviewers:
+ jochen@chromium.org, xhwang@chromium.org

[msw, 2015-06-09 21:53:40]

TBR: jochen for components and xhwang for media.

[msw, 2015-06-09 21:54:16]

The CQ bit was checked by msw@chromium.org

[msw, 2015-06-09 21:54:17]

The patchset sent to the CQ was uploaded after l-g-t-m from sky@chromium.org
Link to the patchset: https://codereview.chromium.org/1175693002/#ps60001
(title: "Restore build/config/ui.gni import for core_services.")

[commit-bot: I haz the power, 2015-06-09 21:58:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1175693002/60001

[commit-bot: I haz the power, 2015-06-09 22:49:52]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-06-09 22:50:35]

Patchset 4 (id:??) landed as
https://crrev.com/4ece34be265cc6205eea813b6805e1659c3131ca
Cr-Commit-Position: refs/heads/master@{#333600}