ASSERTION FAILED: !node || node->isShadowRoot() in WebCore::EventRetargeter::eventTargetRespectingTargetRules

Fix a potential bad cast in EventRetargeter::eventTargetRespectingTargetRules by explicitly checking
that the root node is shadow root before casting.

BUG=263386
TEST=None since testcase does not crash anymore on trunk and is not minimizable.

Review URL: https://chromiumcodereview.appspot.com/21926002

git-svn-id: svn://svn.chromium.org/blink/trunk@155453 bbb929c8-8fbe-4397-9dbb-9b2b20218538