navigator.mediaCapabilities.decodingInfo(...) will now query
the media layer to parse the mime type and check platform support.

Some work remains to reject ambiguous mime strings. In some cases
we are also not deeply checking for platform support. Will get to
this soon.

BUG=695264
TEST=new browser tests

Review-Url: https://codereview.chromium.org/2805553004
Cr-Commit-Position: refs/heads/master@{#467184}