Use --incremental when dex'ing (when possible).

Adds --incremental to dex.py, and a GN flag for enabling it.

Timings for the dex.py step of chrome_java with one file changed:
Before:
real    0m8.068s
user    0m46.996s
sys     0m3.553s

After:
real    0m1.854s
user    0m3.074s
sys     0m0.237s

BUG=533442
Committed: https://crrev.com/e000f9b73cadf25277b401b5a6b27f215aaba1ad
Cr-Commit-Position: refs/heads/master@{#350750}

[agrieve, 2015-09-24 17:18:09]

agrieve@chromium.org changed reviewers:
+ yfriedman@chromium.org

[Yaron, 2015-09-24 19:29:16]

lgtm

[agrieve, 2015-09-25 01:03:36]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-09-25 01:04:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1361703005/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1361703005/40001

[commit-bot: I haz the power, 2015-09-25 02:31:29]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-09-25 02:32:42]

Patchset 3 (id:??) landed as
https://crrev.com/e000f9b73cadf25277b401b5a6b27f215aaba1ad
Cr-Commit-Position: refs/heads/master@{#350750}