Use a reference to avoid a copy of Vector<Node*> in ShadowDOMSiblingTraversalStrategy's constructor.

Looks like a lifetime of Vector<Node*> siblings is longer than that of ShadowDOMSiblingTraversalStrategy.

Note that I couldn't find any *significant* performance gain in PerformanceTest/ShadowDOM/*.hmtl by this change, though.

BUG=None

Review URL: https://codereview.chromium.org/26658002

git-svn-id: svn://svn.chromium.org/blink/trunk@159224 bbb929c8-8fbe-4397-9dbb-9b2b20218538