Add tools/flatten to flatten nested pictures in .skps.

Add tools/flatten to flatten nested pictures in .skps.

Tested by running on skps/sp_desk_nytimes.skp.
The output .skp had no nested draw picture calls, and the files were both 9.3M.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/60c047557559e5410063452e59d65377e813e5f2

[mtklein_C, 2015-07-08 16:02:31]

mtklein@chromium.org changed reviewers:
+ rmistry@google.com

[rmistry, 2015-07-08 16:49:35]

lgtm

[mtklein_C, 2015-07-08 16:53:27]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-07-08 16:53:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1221303020/1

[commit-bot: I haz the power, 2015-07-08 16:55:09]

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

[commit-bot: I haz the power, 2015-07-08 16:55:10]

Try jobs failed on following builders:
  Build-Ubuntu-Clang-x86_64-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu-Clang-x...)

[mtklein_C, 2015-07-08 16:57:58]

The CQ bit was checked by mtklein@chromium.org

[mtklein_C, 2015-07-08 16:57:58]

The patchset sent to the CQ was uploaded after l-g-t-m from rmistry@google.com
Link to the patchset: https://codereview.chromium.org/1221303020/#ps20001
(title: "good catch clang")

[commit-bot: I haz the power, 2015-07-08 16:58:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1221303020/20001

[commit-bot: I haz the power, 2015-07-08 17:06:04]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/60c047557559e5410063452e59d65377e813e5f2