Consider merging https://chromium.googlesource.com/chromium/blink/+/a7dedf81eb7008276bb6854f0e46465e039788f8 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.
This was integrated in WebKit into r160774 already on 18.12.2013. Closing as resolved.