CSS: Drop the quirks-mode exception for CSS MIME types.

      This matches Firefox's behavior (though it returns 'transparent' instead of
'rgba(0, 0, 0, 0)', which is a bit annoying).

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This is a reland of https://codereview.chromium.org/733993002, which was
reverted after breaking tests. https://codereview.chromium.org/1136493003
fixes those tests by sending a correct MIME type for CSS files. The patch
is otherwise identical % rebasing.
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BUG=419383

Review URL: https://codereview.chromium.org/1126133003

git-svn-id: svn://svn.chromium.org/blink/trunk@195266 bbb929c8-8fbe-4397-9dbb-9b2b20218538