ENG-6472 Added histogram latency stats to web studio

This has the issue that when you check if the vaue from the cache is null, you don't retain the reference. What you want is to assign the value of the cache to a variable on the stack, and if it isn't null return that. If it is null, construct the value on to the stack variable, then create the weak reference. 

byte[] retval = null;
if (m_serializedCache == null || (retval = m_serializedCache.get()) == null) {
    retval = m_histogramSupplier.get().toUncompressedBytes();
    m_serializedCache = new WeakReference<byte[]>(retval);
}
return retval;