Combine CENC 'pssh' box parsing routines.

Also update the routines to ignore 'pssh' boxes with version 2 or
later.

BUG=460359, 460360
TEST=new unittests pass

Committed: https://crrev.com/b666d7874efac44b359a95329f0cb890e97671df
Cr-Commit-Position: refs/heads/master@{#333556}

Committed: https://crrev.com/7a84443597e92a03f90806712f8629df40bf408e
Cr-Commit-Position: refs/heads/master@{#333611}

Review URL: https://codereview.chromium.org/1149023002

Cr-Commit-Position: refs/heads/master@{#334416}
9 files changed