EME: Do not special case an empty robustness string.
EME: Do not special case an empty robustness string.

From the EME spec: "The empty string indicates that any ability to decrypt and
decode the content type is acceptable."

Retain a temporary exception for Chrome OS.

BUG=482277,577777

Review URL: https://codereview.chromium.org/1585083003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369690}

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