Introduce SHARED_BUFFER_STATS
Introduce SHARED_BUFFER_STATS

This CL adds some basic profiling instrumentation for SharedBuffer so we can
better understand how SharedBuffer is using memory. When you define the
SHARED_BUFFER_STATS, we'll track all the SharedBuffers in a hash set.
Periodically we dump the contents of the set so that you can see how memory
is being used by SharedBuffers.

R=pdr

Review URL: https://chromiumcodereview.appspot.com/16605004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@152388 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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