Move FontSelector to StyleEngine.

FontSelector is effectively a font cache for the style engine. Move it where it belongs.

As a side benefit, the lifetime relationships between Document and CSSFontSelector are
somewhat simplified.

BUG=291287
R=abarth, eae, eseidel

Review URL: https://chromiumcodereview.appspot.com/23464076

git-svn-id: svn://svn.chromium.org/blink/trunk@158037 bbb929c8-8fbe-4397-9dbb-9b2b20218538