Add Polymer config to enable native shadow dom.

Add Polymer config to enable native shadow dom.

BUG=

Committed: https://crrev.com/b4f819846828f1e16708292713fda665459fd462
Cr-Commit-Position: refs/heads/master@{#329505}

[Jeremy Klein, 2015-05-12 20:49:13]

jlklein@chromium.org changed reviewers:
+ michaelpg@chromium.org

[michaelpg, 2015-05-12 20:51:23]

lgtm

[Jeremy Klein, 2015-05-12 20:52:58]

The CQ bit was checked by jlklein@chromium.org

[commit-bot: I haz the power, 2015-05-12 20:53:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128723004/1

[commit-bot: I haz the power, 2015-05-12 21:06:30]

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

[commit-bot: I haz the power, 2015-05-12 21:06:31]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Jeremy Klein, 2015-05-12 21:10:54]

The CQ bit was checked by jlklein@chromium.org

[Jeremy Klein, 2015-05-12 21:10:55]

The patchset sent to the CQ was uploaded after l-g-t-m from
michaelpg@chromium.org
Link to the patchset: https://codereview.chromium.org/1128723004/#ps20001
(title: "license")

[commit-bot: I haz the power, 2015-05-12 21:11:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1128723004/20001

[commit-bot: I haz the power, 2015-05-12 22:17:48]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-12 22:18:39]

Patchset 2 (id:??) landed as
https://crrev.com/b4f819846828f1e16708292713fda665459fd462
Cr-Commit-Position: refs/heads/master@{#329505}