buffer: report proper retained size in profiler
buffer: report proper retained size in profiler

Make buffers report the proper retained size in heap snapshots.

Before this commit, Buffer objects would show up in the heap profiler as being
only a few hundred bytes large, even if the actual buffer was many megabytes.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node