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This CL moves protocolExtensionManager from SessionConnector into its own class.
The ProtcolExtensionManager will be exposed via the ClientPlugin.

The higher level goal is to remove functionality from the SessionConnecctor
so that it can be removed altogether in a future CL.

BUG=474766

Review URL: https://codereview.chromium.org/1067133002

Cr-Commit-Position: refs/heads/master@{#324167}