Make gyp_flag_compare.py regenerate and take a target name to make it more usable

Make gyp_flag_compare.py regenerate and take a target name to make it more usable

Used to take two text files in obscure ninja format, now
expects the targets to compare. e.g.

tools/gn/bin/gyp_flag_compare.py base

or

tools/gn/bin/gyp_flag_compare.py breakpad_processor_support processor_support

TBR=brettw@chromium.org
BUG=335824
Committed: https://crrev.com/ffcb7f1e0918accebac140f1bdacdfb9c802605c
Cr-Commit-Position: refs/heads/master@{#294111}

[scottmg, 2014-09-10 03:48:34]

scottmg@chromium.org changed reviewers:
+ brettw@chromium.org
- ajwong@chromium.org

[scottmg, 2014-09-10 04:03:56]

(nothing/no one else is using this so I'm just going to check it in)

[scottmg, 2014-09-10 04:04:12]

The CQ bit was checked by scottmg@chromium.org

[commit-bot: I haz the power, 2014-09-10 04:05:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/scottmg@chromium.org/559853002/1

[commit-bot: I haz the power, 2014-09-10 05:26:00]

Committed patchset #1 (id:1) as 86fbb0c99c1684180c78c377a5118b6d0511c218

[commit-bot: I haz the power, 2014-09-10 05:29:20]

Patchset 1 (id:??) landed as
https://crrev.com/ffcb7f1e0918accebac140f1bdacdfb9c802605c
Cr-Commit-Position: refs/heads/master@{#294111}