Remove hasSyntacticallyValidCSSHeader.

This existed apparently to track if a CSS file had at least
a valid CSS rule at the start to attempt to mitigate cross
origin CSS attacks where a file with an invalid mime was
sniffed as CSS and then we skipped the first part which
was not valid, but then interpreted the latter half of
the file as valid CSS.

In Sky all content is same origin, so we don't need this
mitigation.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/800083003