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The code in ParsedContentType is based on RFC2045, which does not mention what
should happen if the same parameter name is mentioned multiple times. However,
RFC6838 (Media Type Specifications and Registration Procedures), which defines
the MIME types for media and is based on RFC2045/2046, has the following
statement:
  It is an error for a specific parameter to be specified more than once.

Add an option to have ParsedContentType fail if the same parameter is
specified multiple times. navigator.requestMediaKeySystemAccess() is updated
to reject MIME types that mention the same parameter name more than once.

BUG=698436
TEST=new tests pass

Review-Url: https://codereview.chromium.org/2736283002
Cr-Commit-Position: refs/heads/master@{#456167}