SkTextBlob plumbing

Add SkTextBlob serialization + drawTextBlob() overrides.

R=mtklein@google.com, reed@google.com, robertphillips@google.com
BUG=269080

Author: fmalita@chromium.org

Review URL: https://codereview.chromium.org/499413002
diff --git a/include/utils/SkDeferredCanvas.h b/include/utils/SkDeferredCanvas.h
index 89cb145..907cb91 100644
--- a/include/utils/SkDeferredCanvas.h
+++ b/include/utils/SkDeferredCanvas.h
@@ -187,6 +187,8 @@
                                 SkScalar constY, const SkPaint&) SK_OVERRIDE;
     virtual void onDrawTextOnPath(const void* text, size_t byteLength, const SkPath& path,
                                   const SkMatrix* matrix, const SkPaint&) SK_OVERRIDE;
+    virtual void onDrawTextBlob(const SkTextBlob* blob, SkScalar x, SkScalar y,
+                                const SkPaint& paint) SK_OVERRIDE;
     virtual void onDrawPatch(const SkPoint cubics[12], const SkColor colors[4],
                              const SkPoint texCoords[4], SkXfermode* xmode,
                              const SkPaint& paint) SK_OVERRIDE;