Fix and cleanup Mojo and Mandoline python scripts.

Fix and cleanup Mojo and Mandoline python scripts.

Simplify the Config class and consolidate init logic.
  Inline GN argument file parsing and make it optional.
  Take an optional build_dir argument for Paths use.

Simplify the Paths class; respect Config's build_dir.
  Only reconstruct build_dir if Config doesn't have one.
  Remove unused methods; rename mojo_runner path member.

Remove unused GN utilities and dart apptest support.

BUG=486220, 488128
TEST=android_run_mandoline.py, android_mojo_shell.py, and apptest_runner.py work as expected.
R=sky@chromium.org

Committed: https://crrev.com/6c8ec83b6163f32361ca742e91189969f759ed7f
Cr-Commit-Position: refs/heads/master@{#330391}

[msw, 2015-05-15 22:39:45]

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

[msw, 2015-05-15 23:00:21]

Hey Scott, please take a look; thanks!

[sky, 2015-05-18 17:05:16]

LGTM

[msw, 2015-05-18 17:54:50]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2015-05-18 17:55:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1144673003/40001

[commit-bot: I haz the power, 2015-05-18 20:16:41]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-05-18 22:41:30]

Patchset 3 (id:??) landed as
https://crrev.com/6c8ec83b6163f32361ca742e91189969f759ed7f
Cr-Commit-Position: refs/heads/master@{#330391}