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This CL converts ExtensionMsg_SetSessionInfo to mojo
and introduces channel_mojom_traits.h to convert
version_info::Channel to extensions.mojom.Channel.

Bug: 1146103
Change-Id: Iacc3d116bf08a7f32eadb5066cd9690b3aec89ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548943
Commit-Queue: Julie Kim <jkim@igalia.com>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829934}