Remove reference counting of WebHelperPluginImpl.

WebMediaPlayerClientImpl has a RefPtr<> to WebHelperPluginImpl, which is changed to OwnPtr<> to make clear the ownership of the plugin (and remove the reference counting). Also adds a customized destructor for OwnPtr<WebHelperPluginImpl> so that it gets cleaned up in the correct order.

BUG=336502
TEST=EncryptedMedia browser_tests succeed

Review URL: https://codereview.chromium.org/141003007

git-svn-id: svn://svn.chromium.org/blink/trunk@165680 bbb929c8-8fbe-4397-9dbb-9b2b20218538