Added a plugin directory for OSP.

Added a plugin directory for OSP.

[Title] Added a plugin directory for OSP.
[Issue#] N/A
[Problem] OSP plugin does not work.
[Cause] WKContextSetAdditionalPluginsDirectory() is not work properly because
        it sets additional plugin directory of only one PluginInfoStore.
        Every WebContext makes its own PluginInfoStore. But it's not possible
        to set the value of every PluginInfoStore.
[Solution] Add the OSP plugin directory to default plugin directories.
[Developer] ky0.choi

Change-Id: If3bb5dc43f8d13cf5d4713c104a215e8802f8f94