MSE: Consistently sequence initial StreamParser EncryptedMediaInitDataCB
MSE: Consistently sequence initial StreamParser EncryptedMediaInitDataCB

Comparing webm_stream_parser.cc and mp4_stream_parser.cc, the former
emits the initial EncyrptedMediaInitDataCB prior to the first NewConfigCB, while
the latter has reverse sequence.

This change makes the ordering consistent across these two parsers.

R=xhwang@chromium.org

Review URL: https://codereview.chromium.org/1583733002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369252}

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