Prohibit createShadowRoot({mode: 'open'}) on existing shadow root.

Prohibit createShadowRoot({mode: 'open'}) on existing shadow root.

This implements Shadow DOM spec 8.2.2-2.
https://w3c.github.io/webcomponents/spec/shadow/#widl-Element-createShadowRoot-ShadowRoot-ShadowRootInit-shadowRootInitDict
> createShadowRoot
...
> 2. If the context object already hosts the shadow tree, throws InvalidStateError exception.

Calling createShadowRoot() without init dictionary to create multple shadow roots are
still allowed for compatibility.

BUG=459136
TEST=fast/dom/shadow/create-shadow-root-with-parameter.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198388

[kochi, 2015-07-06 08:03:26]

The CQ bit was checked by kochi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-06 08:03:32]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1223473004/1

[kochi, 2015-07-06 08:05:53]

kochi@chromium.org changed reviewers:
+ hayato@chromium.org

[kochi, 2015-07-06 08:06:28]

PTAL

[commit-bot: I haz the power, 2015-07-06 09:30:13]

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

[commit-bot: I haz the power, 2015-07-06 09:30:14]

Dry run: This issue passed the CQ dry run.

[hayato, 2015-07-07 08:46:51]

lgtm

[kochi, 2015-07-07 09:32:10]

The CQ bit was checked by kochi@chromium.org

[commit-bot: I haz the power, 2015-07-07 09:32:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1223473004/1

[commit-bot: I haz the power, 2015-07-07 10:36:46]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198388