Simplify Node::updateDistribution().
Simplify Node::updateDistribution().

parentNode() is enough here because |root| is already on the *highest* shadow host.

BUG=None
TEST=None; No behavior change

Review URL: https://codereview.chromium.org/1033163003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192660 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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