Measure usage of the prefixed encrypted media APIs

The unprefixed replacements for these aren't implemented yet, but it's
just as well to start collecting data early.

PrefixedMediaGenerateKeyRequest and PrefixedMediaAddKey were already in
UseCounter.h, but unused. Add the missing PrefixedMediaCancelKeyRequest
and put them all to use.

Remove the deprecation message for PrefixedMediaGenerateKeyRequest,
which was left behind in r150962. Otherwise, the ASSERTs in
UseCounter::count would start failing.

BUG=334983

Review URL: https://codereview.chromium.org/140663002

git-svn-id: svn://svn.chromium.org/blink/trunk@165290 bbb929c8-8fbe-4397-9dbb-9b2b20218538