Remove HelperPlugin code from WebMediaPlayerClient

Now that the helper plugin can be loaded outside Blink, the
createHelperPlugin()/closeHelperPluginSoon() methods are no
longer needed.

BUG=336520
TEST=EME content tests pass

Review URL: https://codereview.chromium.org/179203003

git-svn-id: svn://svn.chromium.org/blink/trunk@168063 bbb929c8-8fbe-4397-9dbb-9b2b20218538