Add request cache to SkFontHost_fontconfig.

The current code deduplicates SkTypeface instances as all font lookups
should for better use of the glyph cache. This adds a request cache as
well, so that repeated recent requests will return the cached result
instead of doing a full lookup.

BUG=chromium:424082, chromium:444894

Review URL: https://codereview.chromium.org/1683883002
3 files changed