12 RFR(S) 8215205: javaVFrame much slower than vframeStream

[dean.long at oracle.com]

https://bugs.openjdk.java.net/browse/JDK-8215205
http://cr.openjdk.java.net/~dlong/8215205/webrev

This change allows a stackwalk to continue using the faster vframeStream 
iterator, but changing to a javaVFrame lazily as needed.  It introduces 
a new API

   javaVFrame* vframeStreamCommon::asJavaVFrame()


The first user of this API will be JDK-8214329, which is why this issue 
is a P3.

dl