Remove the WEB_AUDIO compile time flag

Web audio already has a runtime feature flag,
so the WEB_AUDIO compile-time flag wouldn't be needed.
Per the comment in https://codereview.chromium.org/604753003/#msg8,
it looks safe to remove the compile-time flag.

Web audio is the last feature that uses a compile-time flag.
Once we remove WEB_AUDIO, we can remove a lot of code to support
compile-time features from the IDL comipler.

BUG=417628
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/1557363002

Cr-Commit-Position: refs/heads/master@{#368554}
228 files changed