Convert files with only CRLF line endings to pure LF

WebIDL/tests/submissions/W3C/valid/idl/documentation-dos.widl was
left as CRLF because the name makes it look intentional.

3 files with mixed line endings remain:

cors/resources/cors-headers.asis
html/semantics/embedded-content-0/the-canvas-element/size.attributes.parse.whitespace.html
old-tests/submission/Opera/media/track/webvtt/parsing/support/newlines.vtt

To verify that this commit makes no other changes:

$ git diff --name-only HEAD^ | xargs unix2dos
$ git diff HEAD^ # should show nothing