SkPDF: Simplify PDFStream / emitObject() const

Compress SkPDFStream's data on setData(), not emitObject(); no longer stateful.

SkPDFObject::emitObject is now const.  This makes it easier to reason about state.

Minimal performance gains.

Review URL: https://codereview.chromium.org/1304493002
diff --git a/src/pdf/SkPDFGraphicState.h b/src/pdf/SkPDFGraphicState.h
index 1b11fe4..45a32d4 100644
--- a/src/pdf/SkPDFGraphicState.h
+++ b/src/pdf/SkPDFGraphicState.h
@@ -32,9 +32,9 @@
 
     // Override emitObject so that we can populate the dictionary on
     // demand.
-    virtual void emitObject(SkWStream* stream,
-                            const SkPDFObjNumMap& objNumMap,
-                            const SkPDFSubstituteMap& substitutes);
+    void emitObject(SkWStream* stream,
+                    const SkPDFObjNumMap& objNumMap,
+                    const SkPDFSubstituteMap& substitutes) const override;
 
     /** Get the graphic state for the passed SkPaint. The reference count of
      *  the object is incremented and it is the caller's responsibility to