Introduce CSSSegmentedFontFaceCache.

Out of hashes of caches, from the bowels CSSFontSelector, a new class is born.
Its responsibility is to vend and hold the CSSSegmentedFontFace instances.

BUG=291287
R=eae, eseidel, esprehn

Review URL: https://codereview.chromium.org/25802003

git-svn-id: svn://svn.chromium.org/blink/trunk@158889 bbb929c8-8fbe-4397-9dbb-9b2b20218538