Remove declaration of CSSSelector::setMatchUserAgentOnly.
Remove declaration of CSSSelector::setMatchUserAgentOnly.

The implementation of this was removed over a year ago:
  https://codereview.chromium.org/82863002/

BUG=321953

Review URL: https://codereview.chromium.org/727863003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@185886 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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