Modifying TextBlobCacheTest to use SkRandomScalerContext

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/adcdca86ce425cf8c28bfad311cef028df756ee8

Review URL: https://codereview.chromium.org/1266003002
diff --git a/src/gpu/GrBatchFontCache.cpp b/src/gpu/GrBatchFontCache.cpp
index d6ffc50..64d6bb1 100644
--- a/src/gpu/GrBatchFontCache.cpp
+++ b/src/gpu/GrBatchFontCache.cpp
@@ -226,6 +226,7 @@
                                                glyph->width(), glyph->height(),
                                                storage.get(), &glyph->fAtlasLocation);
     if (success) {
+        SkASSERT(GrBatchAtlas::kInvalidAtlasID != glyph->fID);
         fAtlasedGlyphs++;
     }
     return success;