WebPReportProgress: use non-encoder specific params

Take picture and percent value storage location instead of VP8Encoder.
This will allow reuse by the lossless encoder.

Change-Id: Ic49dbc800cc3e2df60d20f4ebac277f68ed6031b

src/enc/frame.c

@@ -929,7 +929,7 @@ int VP8StatLoop(VP8Encoder* const enc) {
       }
     }
   }
-  return WebPReportProgress(enc, final_percent);
+  return WebPReportProgress(enc->pic_, final_percent, &enc->percent_);
 }
 
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