SVGLength::SetValueAsString was allowing "" to "reset" the value without
signaling an error. Because of how the handling of removeAttribute works
we can't shut down this entirely, but still need to handle the null
string resulting from that.

Convert svg/custom/invalid-length-units.html to testharness.js. (This
test had one item for "" and " ".)

Bug: 779454
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I055b77b07d833a252b325df8fc1a908271c86005
Reviewed-on: https://chromium-review.googlesource.com/746707
Reviewed-by: Philip Rogers <[email protected]>
Commit-Queue: Fredrik Söderquist <[email protected]>
Cr-Commit-Position: refs/heads/master@{#512871}