Move createHelperPlugin() to WebView

This allows non-Blink code to create and manage the lifetime of helper plugins without having to callback into Blink code to get it done.

BUG=336520
TEST=EME content tests pass

Review URL: https://codereview.chromium.org/149523005

git-svn-id: svn://svn.chromium.org/blink/trunk@166663 bbb929c8-8fbe-4397-9dbb-9b2b20218538