Enable WebAudio mp3 codec tests

Mp3 codecs are available in Chromium now, so enable these webaudio mp3
tests.

Bug: 822797
Change-Id: I901dc28db882cbdaac1a74c28e0cef91ced32162
Reviewed-on: https://chromium-review.googlesource.com/966924Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543840}

third_party/WebKit/LayoutTests/NeverFixTests

@@ -275,8 +275,7 @@ crbug.com/627953 [ Android Linux Mac ] fast/text/symbol-cmap.html [ WontFix ]
 crbug.com/642912 http/tests/wasm/ [ WontFix ]
 
 # These tests require audio codecs which are generally not available;
-# these tests can still be run manually with --skiped=ignore.
-webaudio/codec-tests/mp3 [ WontFix ]
+# these tests can still be run manually with --skipped=ignore.
 webaudio/codec-tests/aac [ WontFix ]
 
 # WPT for CSS cascade