RFR(XS): 8264562: assert(verify_field_bit(1)) failed: Attempting to write an uninitialized event field: type

[Yangfei (Felix)]

Hi,

Please review this one-line change fixing a JFR crash for 8u: 
Bug:
    https://bugs.openjdk.java.net/browse/JDK-8264562 
    http://cr.openjdk.java.net/~fyang/8264562/webrev.00/ 

This issue can always be reproduced by running the following two jtreg tests with 8u debug build: 
    jdk/jfr/event/gc/collection/TestGCEventMixedWithG1ConcurrentMark.java 
    jdk/jfr/event/gc/collection/TestGCEventMixedWithG1FullCollection.java 

I think the reason is that we are not setting trace type for EventG1HeapRegionInformation event. 
Patch adds back "type" information for EventG1HeapRegionInformation event. 
This should also improves basic functionality of 8u JFR EventG1HeapRegionInformation tracing. 

Performed full jtreg test with fastdebug build on x86_64 Linux. 
These two affected jtreg tests fail without this fix and pass otherwise. 
OK? 

Thanks,
Felix