Split SkFontConfigInterface globals and factory.

Chromium needs to be able to set up their build such that the globals
continue existing but the SkFontMgr::Factory can be defined separately.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2346333002
CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac-Clang-x86_64-Release-CMake-Trybot

Review-Url: https://codereview.chromium.org/2346333002
9 files changed