Fix closure compiler error.
Fix closure compiler error.

Fixes compile error caused by https://codereview.chromium.org/1425033006.
Although AudioPlayerElement's new property can be resolved by reading element's script directly,
I fixes this in old style (using extern) for now.

BUG=none
TBR=yawano@chromium.org

Review URL: https://codereview.chromium.org/1428983002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357120}

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