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SVGRenderSupport::layoutChildren() determines whether the size of the
nearest viewport has changed upfront, and then uses this information to
force a re-layout of relative-length children.

But the relative-length flag is always propagated up the ancestor chain
(see SVGElement::updateRelativeLengthsInformation), so we can
immediately tell whether there are any relative-length descendants. If
there aren't, we can skip the viewport change crawl and related logic.

R=pdr@chromium.org,schenney@chromium.org,fs@opera.com,ed@opera.com

Review URL: https://codereview.chromium.org/458833002

git-svn-id: svn://svn.chromium.org/blink/trunk@180071 bbb929c8-8fbe-4397-9dbb-9b2b20218538