Rewrote perform_action_on_all_files to be parallell.

Rewrote perform_action_on_all_files to be parallell.

This speeds up decode for a 720p workload from 25s to 5s on my machine.
I did some benchmarking and it appears we spend the vast majority of
the time on zxing decoding, so now we will bring all processors on the
the machine to bear.

BUG=webrtc:4986
Committed: https://crrev.com/be81fa538d915542c0be7a304398aa60b4e3de67
Cr-Commit-Position: refs/heads/master@{#10443}

[phoglund, 2015-10-26 15:50:31]

phoglund@webrtc.org changed reviewers:
+ kjellander@webrtc.org

[kjellander_webrtc, 2015-10-28 17:53:59]

lgtm, awesome!

[phoglund, 2015-10-29 08:14:19]

The CQ bit was checked by phoglund@webrtc.org

[commit-bot: I haz the power, 2015-10-29 08:14:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1428433002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1428433002/1

[commit-bot: I haz the power, 2015-10-29 09:22:50]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-29 09:22:58]

Patchset 1 (id:??) landed as
https://crrev.com/be81fa538d915542c0be7a304398aa60b4e3de67
Cr-Commit-Position: refs/heads/master@{#10443}