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* Zero out |total_size_| member in SpdyReadQueue::Clear(), otherwise the next
  call to IsEmpty() would crash.
* Add test for SpdyReadQueue::Clear().
* Add a comment to document class invariant.
* Move SpdyReadQueue tests into namespace test.

This is a follow-up to
https://crrev.com/2386883002/diff/40001/net/spdy/spdy_read_queue.cc#newcode53

Review-Url: https://codereview.chromium.org/2388243003
Cr-Commit-Position: refs/heads/master@{#422875}