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Most of the tests are migrated from CastMessageHandlerTest. Some
new tests are added as CafMessageHandler has more methods than
CastMessageHandler:

* testHandleClientConnectMessage*
* testHandleClientDisconnectMessage*
* testHandleClientLeaveSessionMessage*
* testSendReceiverActionToClient
* testBuildSessionMessage*

Bug: 711860
Change-Id: I821bb91f0c685809c143b60cc4700c47ffec16b6
Reviewed-on: https://chromium-review.googlesource.com/c/1343292
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609805}