Remove TODOs from lossless encoder code.

histogram.c:
 - Verified (earlier) that there's low correlation between Red & Blue colors
   (particularly after applying Cross-color transform). The Bin based histogram
   merge, bins on three entropies viz literal, red & blue symbols. Removing
   either of blue or red increases the compression density. So keeping the bins
   for red & blue sybmols.
 - Keeping the compact bins method as-is. This way it's simpler to read.
huffman_encode.h: Added field comments for struct HuffmanTree and removed the TODO.

Change-Id: Ia76f7bc730079d1b3b644038c5d9931db3797f0e

src/enc/histogram.c

@@ -435,7 +435,6 @@ static int GetBinIdForEntropy(double min, double max, double val) {
   return (int)(NUM_PARTITIONS * delta / range);
 }
 
-// TODO(vikasa): Evaluate, if there's any correlation between red & blue.
 static int GetHistoBinIndex(
     const VP8LHistogram* const h, const DominantCostRange* const c) {
   const int bin_id =
@@ -524,8 +523,6 @@ static void HistogramAnalyzeEntropyBin(
 // Compact the histogram set by moving the valid one left in the set to the
 // head and moving the ones that have been merged to other histograms towards
 // the end.
-// TODO(vikasa): Evaluate if this method can be avoided by altering the code
-// logic of HistogramCombineEntropyBin main loop.
 static void HistogramCompactBins(VP8LHistogramSet* const image_histo) {
   int start = 0;
   int end = image_histo->size - 1;

src/utils/huffman_encode.h

@@ -34,10 +34,9 @@ typedef struct {
 } HuffmanTreeCode;
 
 // Struct to represent the Huffman tree.
-// TODO(vikasa): Add comment for the fields of the Struct.
 typedef struct {
-  uint32_t total_count_;
-  int value_;
+  uint32_t total_count_;   // Symbol frequency.
+  int value_;              // Symbol value.
   int pool_index_left_;    // Index for the left sub-tree.
   int pool_index_right_;   // Index for the right sub-tree.
 } HuffmanTree;