Fix ChromotingInstance to use ChromotingConnection only after Start().
Fix ChromotingInstance to use ChromotingConnection only after Start().

Previously ChromotingInstance::HandleConnect() was calling
client_->input_stub() before Start(). input_stub() should not be called
before Start() because it may not be initialized at that point.

BUG=582518

Review URL: https://codereview.chromium.org/1674223003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374233}

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