Refactor SkGrFontScaler and SkGrFontKey into non-virtual versions.

This is a pre-cleanup for another change, but has the side benefit of making
the code simpler in general.

No perf increase, despite removing virtual functions.

R=bsalomon@google.com, egdaniel@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/385263002
diff --git a/src/gpu/GrTextStrike.cpp b/src/gpu/GrTextStrike.cpp
index a9405ca..b290d6c 100644
--- a/src/gpu/GrTextStrike.cpp
+++ b/src/gpu/GrTextStrike.cpp
@@ -220,7 +220,7 @@
     atlas and a position within that texture.
  */
 
-GrTextStrike::GrTextStrike(GrFontCache* cache, const GrKey* key,
+GrTextStrike::GrTextStrike(GrFontCache* cache, const GrFontDescKey* key,
                            GrMaskFormat format,
                            GrAtlas* atlas) : fPool(64) {
     fFontScalerKey = key;