Should never be reached failure in WebCore::floatValueForLength

Should never be reached failure in WebCore::floatValueForLength
https://bugs.webkit.org/show_bug.cgi?id=139397
<rdar://problem/27704376>

Reviewed by Simon Fraser.

Source/WebCore:

floatValueForLength can't resolve unspecified Length types. Filter them out and return 0 as if they were auto.

Test: svg/css/assert-on-non-resolvable-dimension.html

* svg/SVGLengthContext.cpp:
(WebCore::SVGLengthContext::valueForLength):

LayoutTests:

* svg/css/assert-on-non-resolvable-dimension-expected.txt: Added.
* svg/css/assert-on-non-resolvable-dimension.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@205392 268f45cc-cd09-0410-ab3c-d52691b4dbfc