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This change moves all the encrypted media code out of WebMediaPlayerImpl
and into a EncryptedMediaSupportImpl class. This removes the encrypted
media related content/ depencencies from WebMediaPlayerImpl so that it
will be easier to move WebMediaPlayerImpl to a location where it can
be reused by Mojo.

Review URL: https://codereview.chromium.org/501473003

Cr-Commit-Position: refs/heads/master@{#291776}