RFR JDK-8247247: JFR tests fail due to JDK-8235521 missing doPrivileged block

[Mandy Chung]

Webrev:
http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8247247/webrev.00/

The call to `Unsafe::ensureClassInitialized` was replaced with 
`MethodHandles.privateLookupIn(clazz, LOOKUP).ensureInitialized(clazz)` 
by JDK-8235521 but missing the doPrivileged block.

The call to `MethodHandles::privateLookupIn` has an unconditional 
security permission check whereas the call to 
`Lookup::ensureInitialized` may trigger a permission check if the Lookup 
object is not a full privilege Lookup.  The fix is to only wrap 
``MethodHandles::privateLookupIn` with a doPrivileged block and keep 
Lookup::ensureInitialized` call without doPrivileged that was the 
previous behavior.  This fix also ensures that the class is initialized 
only at event class registration time which is done with doPrivileged 
block (i.e. security check on Lookup::ensureInitialized will pass). 
OTOH, getEventType and unregister are changed not to initialize the class.

Thanks
Mandy