Speed-up: Make sure we only initialize histograms when needed.

Also, histograms in a HistogramSet can be initialized all
at once.

Change-Id: Ibbfa6034dce58dca8bb9113487e2ae507222ce7d

src/enc/vp8l_enc.c

@@ -809,6 +809,7 @@ static WebPEncodingError EncodeImageNoHuffman(VP8LBitWriter* const bw,
     err = VP8_ENC_ERROR_OUT_OF_MEMORY;
     goto Error;
   }
+  VP8LHistogramSetClear(histogram_image);
 
   // Build histogram image and symbols from backward references.
   VP8LHistogramStoreRefs(refs, histogram_image->histograms[0]);