dec/dsp/enc/utils,cosmetics: rm struct member '_' suffix

This is a follow up to:
ee8e8c62 Fix member naming for VP8LHistogram

This better matches Google style and clears some clang-tidy warnings.

This is the final change in this set. It is rather large due to the
shared dependencies between dec/enc.

Change-Id: I89de06b5653ae0bb627f904fa6060334831f7e3b

tests/fuzzer/huffman_fuzzer.cc

@@ -50,7 +50,7 @@ void HuffmanTest(std::string_view blob) {
   VP8LBitReader* br;
   VP8LDecoder* dec = VP8LNew();
   if (dec == NULL) goto Error;
-  br = &dec->br_;
+  br = &dec->br;
   VP8LInitBitReader(br, data, size);
 
   color_cache_bits = VP8LReadBits(br, kColorCacheBitsBits);