[SVG] invalidateRelativeLengthClients() should also handle root elements

Currently, SVGElement::invalidateRelativeLengthClients() gets called for
all SVGSVGElements in a document when the container size changes. But
the method only invalidates children - so if the SVG element itself has
relative lengths, it never gets invalidated.

The CL moves the invalidation logic out of the children loop, to operate
on the current element instead (children are handled recursively).

BUG=313083
R=pdr@chromium.org,schenney@chromium.org

Review URL: https://codereview.chromium.org/52713002

git-svn-id: svn://svn.chromium.org/blink/trunk@160944 bbb929c8-8fbe-4397-9dbb-9b2b20218538