Refactoring: Break CSSFontFaceSource into several classes

Before this patch, CSSFontFaceSource was handling several different font
sources (local, webfonts, in-document / remote SVG fonts). This patch
breaks CSSFontFaceSource into a class hierarchy consists of the
following classes:

- CSSFontFaceSource: The abstract base class
- LocalFontFaceSource: For src:local()
- RemoteFontFaceSource: For src:url()
- SVGFontFaceSource: In-document SVG fonts
- SVGRemoteFontFaceSource: Remote SVG fonts

BUG=none
TEST=no behavior change, covered by existing tests

Review URL: https://codereview.chromium.org/193923003

git-svn-id: svn://svn.chromium.org/blink/trunk@169186 bbb929c8-8fbe-4397-9dbb-9b2b20218538