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In the current production code, `client_` is always set immediately
after the MIDIDispatcher construction, and there is no timing to run
SessionStarted without a valid `client_` on the same thread.

This SECURITY_CHECK just makes the code robust against attacks via
mojo injections and code changes in the future.

Bug: 1161806
Change-Id: Ic8a5f6e0dc70e6bb4c7cccb4d69d63fc12382c19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631811
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Auto-Submit: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844503}