[inspector_protocol] Introduce protocol::Serializable

sendProtoolResponse and sendProtocolNotification methods report message as protocol::Serializable. So client can decide by itself when it's good time to setialize messages.

BUG=chromium:350797
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/2526603002 .

Committed: https://chromium.googlesource.com/deps/inspector_protocol0c231497bdd29a2c17e04d6e7f4eb1030998a308
14 files changed