Unmark inspector/elements/styles/stylesheet-source-url-comment.html as Slow

The test is no longer slow on Release bots, presumably due to r171565.

TBR=tkent@chromium.org
BUG=357003
NOTRY=true

Review URL: https://codereview.chromium.org/267133003

git-svn-id: svn://svn.chromium.org/blink/trunk@173383 bbb929c8-8fbe-4397-9dbb-9b2b20218538