SkTextBlob: Begin implementing Extended TextBlob API

include/core/SkPicture.h

Index: include/core/SkPicture.h
diff --git a/include/core/SkPicture.h b/include/core/SkPicture.h
index 7cfa6679f3fdc2c96d2bb68c23a1531f44ecad31..9ba851005d59088968629569d193fa493e81560c 100644
--- a/include/core/SkPicture.h
+++ b/include/core/SkPicture.h
@@ -213,10 +213,11 @@ private:
     // V45: Add invNormRotation to SkLightingShader.
     // V46: Add drawTextRSXform
     // V47: Add occluder rect to SkBlurMaskFilter
+    // V48: Read and write extended SkTextBlobs.
 
     // Only SKPs within the min/current picture version range (inclusive) can be read.
     static const uint32_t     MIN_PICTURE_VERSION = 35;     // Produced by Chrome M39.
-    static const uint32_t CURRENT_PICTURE_VERSION = 47;
+    static const uint32_t CURRENT_PICTURE_VERSION = 48;
 
     static_assert(MIN_PICTURE_VERSION <= 41,
                   "Remove kFontFileName and related code from SkFontDescriptor.cpp.");