WebUI: Fix autogenerated JS import paths on Windows

This allows running:
python polymer_test.py from tools\polymer to pass on Windows, and
prevents ..\js\paths\like\this.m.js, which cause a parsing error.

Bug: 965770
Change-Id: I23701cffeab4ef5bf7d61bb06307b6fe8a9e17e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715147
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680142}
1 file changed