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The BlimpClientSession will be going away, so this CL takes part of it
and splits it up into smaller parts and moves them to
//blimp/client/core/session. They will be used by the
BlimpClientContextImpl.

BUG=611097

Review-Url: https://codereview.chromium.org/2187893002
Cr-Commit-Position: refs/heads/master@{#408802}