Clamp argument to SVGGeometryElement.getPointAtLength to [0, length]

Per:

  https://svgwg.org/svg2-draft/types.html#__svg__SVGGeometryElement__getPointAtLength

Bug: 847198
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I9067ce9f196453bd54560d5119ebc38d35c8ac86
Reviewed-on: https://chromium-review.googlesource.com/1075332
Reviewed-by: Stephen Chenney <[email protected]>
Commit-Queue: Fredrik Söderquist <[email protected]>
Cr-Commit-Position: refs/heads/master@{#562422}