[SVG] Fix root element length values handling.

SVGLengthContext::determineViewport() currently asserts that we're not
resolving lengths for the topmost element, but there's nothing to
prevent such calls.

The CL updates determineViewport() to handle root elements geracefully
(using their current viewport). It also changes the signature slightly
to operate directly on a FloatSize, reducing some of the boiler-plate
client code.

BUG=317484
R=pdr@chromium.org, schenney@chromium.org

Review URL: https://codereview.chromium.org/67483003

git-svn-id: svn://svn.chromium.org/blink/trunk@161749 bbb929c8-8fbe-4397-9dbb-9b2b20218538