Remove TODO for mime-sniffing of mpeg types.

The following TODO existed in mime_sniffer.cc since 2008:

// TODO(abarth): we don't handle partial byte matches yet
// MAGIC_NUMBER("video/mpeg", "\x00\x00\x01\xB")
// MAGIC_NUMBER("audio/mpeg", "\xFF\xE")
// MAGIC_NUMBER("audio/mpeg", "\xFF\xF")

video/mpeg is not sniffed by Firefox or IE, and is not required to be sniffed by the spec at https://mimesniff.spec.whatwg.org/.

audio/mpeg is already sniffed for the common case where the file starts with an ID3 tag. The uncommon case where it starts with "\xFF\xF." is too broad (it matches the little-endian UTF-16 byte-order-mark), and is not required by the spec.

Remove the TODO.

TEST=net_unittests
BUG=

Review URL: https://codereview.chromium.org/1138483004

Cr-Commit-Position: refs/heads/master@{#329317}

net/base/mime_sniffer.cc

@@ -162,10 +162,6 @@ static const MagicNumber kMagicNumbers[] = {
   MAGIC_NUMBER("audio/mpeg", "ID3")
   MAGIC_NUMBER("image/webp", "RIFF....WEBPVP8 ")
   MAGIC_NUMBER("video/webm", "\x1A\x45\xDF\xA3")
-  // TODO(abarth): we don't handle partial byte matches yet
-  // MAGIC_NUMBER("video/mpeg", "\x00\x00\x01\xB")
-  // MAGIC_NUMBER("audio/mpeg", "\xFF\xE")
-  // MAGIC_NUMBER("audio/mpeg", "\xFF\xF")
   MAGIC_NUMBER("application/zip", "PK\x03\x04")
   MAGIC_NUMBER("application/x-rar-compressed", "Rar!\x1A\x07\x00")
   MAGIC_NUMBER("application/x-msmetafile", "\xD7\xCD\xC6\x9A")