Add simple font fallback on Mac.
Add simple font fallback on Mac.

Implement SkFontMgr_Mac::onMatchFamilyStyleCharacter. This is a simple
implementation which provides basic fallback.

This also renames and refactors several static functions to reduce code
duplication.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2268663002

Review-Url: https://codereview.chromium.org/2268663002

google/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,rubenvb/skia,rubenvb/skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc