[eme] Simplify parsing of contentTypes

Now that ParsedContentType does case-insensitive parameter lookups
(http://crbug.com/689731), use it to extract the type and codecs
parameter.

BUG=605661
TEST=existing case-insensitive EME tests pass

Review-Url: https://codereview.chromium.org/2728713003
Cr-Commit-Position: refs/heads/master@{#454664}
1 file changed