Zero-initialize all members of EncodedFrame.
Zero-initialize all members of EncodedFrame.

ntp_time_ms_ was missing in the default constructor, these things are
very easy to miss, so adding C++11-style initialization instead. This
also reduces init-list duplication.

BUG=
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44609004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8669}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4c8b93091d9225587d7b78a939225a3d3f7fed66

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