Make SkFont a bit more useable

Split out of https://codereview.chromium.org/2163483002/ (Use SkFont in GrStencilAndCoverTextContext)

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2163633002

Review-Url: https://codereview.chromium.org/2163633002
diff --git a/include/core/SkPaint.h b/include/core/SkPaint.h
index c0f1f3e..caa1856 100644
--- a/include/core/SkPaint.h
+++ b/include/core/SkPaint.h
@@ -1085,7 +1085,9 @@
         uint32_t fBitfieldsUInt;
     };
 
-    GlyphCacheProc getGlyphCacheProc(bool needFullMetrics) const;
+    static GlyphCacheProc GetGlyphCacheProc(TextEncoding encoding,
+                                            bool isDevKern,
+                                            bool needFullMetrics);
 
     SkScalar measure_text(SkGlyphCache*, const char* text, size_t length,
                           int* count, SkRect* bounds) const;