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- Define StringToCodecMap to avoid typing the long name multiple times.
- base::NoDestructor supports emplace; no need to create a map first.
- KEEP_FIRST_OF_DUPES is the default behavior; no need to specify.
- Remove #if BUILDFLAG(ENABLE_AC3_EAC3_AUDIO_DEMUXING), because:
  * It's causing clang format to misbehave.
  * We should keep the mapping simple and should not restrict feature
    here, especially since MimeUtil::Codec::EAC3 is defined
    unconditionally.
- Reformatted by "git cl format"

Bug: 897767
Test: No functionality change.
Change-Id: Iac8d8b8ae5a9a1acbab89105c121f1d80badd536
Reviewed-on: https://chromium-review.googlesource.com/c/1294598Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601734}