Support persistent sessions with CDM5/6.
Support persistent sessions with CDM5/6.

The prefixed API adds some known data to the front of the init_data
parameter to specify persistent sessions. CDM5/6 includes a parameter
to CreateSession() that specifies if the session is persistent or not.
Use the prefix to determine if a session is persistent (and remove it
from the init_data before passing it to the CDM).

BUG=403527
TEST=encrypted-media layout tests and browser_tests for encrypted media pass

Review URL: https://codereview.chromium.org/471763002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#289794}
git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@289794 0039d316-1c4b-4281-b951-d872f2087c98

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