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/src/utils/debugger/SkDebugCanvas.h b/src/utils/debugger/SkDebugCanvas.h
index e4fb0d9..94ad426 100644
--- a/src/utils/debugger/SkDebugCanvas.h
+++ b/src/utils/debugger/SkDebugCanvas.h
@@ -242,6 +242,8 @@
                                 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 onPushCull(const SkRect& cullRect) SK_OVERRIDE;
     virtual void onPopCull() SK_OVERRIDE;