Don't error if NeedsRebaseline is in a comment in TestExpectations.

auto-rebaseline was failing because it was trying to parse a comment
line as a standard format expectations line and it failed. Instead,
strip comments first.

Review URL: https://codereview.chromium.org/41023002

git-svn-id: svn://svn.chromium.org/blink/trunk@160485 bbb929c8-8fbe-4397-9dbb-9b2b20218538