RFR: 8202230: Provide accessors for JNIHandles storage objects

[Kim Barrett]

> On Apr 25, 2018, at 5:34 PM, coleen.phillimore at oracle.com wrote:
> 
> Looks good.
> Coleen

Thanks.

> On 4/25/18 5:09 PM, Kim Barrett wrote:
>> Please review this addition to JNIHandles, providing access to the
>> OopStorage objects used to manage the global and weak global handles.
>> The accessors assert the storage objects have been allocated.
>> 
>> Also changed direct uses of the storage object variables to instead
>> use the new accessors, to get the initialization asserts during
>> development.
>> 
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8202230
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8202230/open.00/
>> 
>> Testing:
>> {jdk,hs}-tier{1,2,3} for all Oracle platforms.