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This CL instruments other objects (e.g. buffers, queues, strings)
referenced from SpdySessionPool and SpdySession. The goal is
to make the memory estimate reported by
SpdySessionPool::DumpMemoryStats() cover more parts of the code.

However, there are still a couple of TODOs because
base/trace_event/memory_usage_estimator.h doesn't yet support
linked_hash_map, scoped_refptr and WeakPtr.

BUG=669108

Review-Url: https://codereview.chromium.org/2665283003
Cr-Commit-Position: refs/heads/master@{#449431}