Reset fontselector() when removing stylesheets and left stylesheets still have @font-face rules.

If not reset, CSSSegmentedFontFaceCache has mulitple fontFaces for the same @font-face rule.
This causes fast/css/font-face-html-as-svg flaky (Timeout in mac-blink-rel).

BUG=305885,309370

Review URL: https://codereview.chromium.org/73993002

git-svn-id: svn://svn.chromium.org/blink/trunk@162432 bbb929c8-8fbe-4397-9dbb-9b2b20218538