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This value has no meaningful usage in the codebase. Its removal has
ripple effects:
- chrome_extension_messages.h is no longer needed and is removed
- cast_extension_messages.{cc, h} are no longer needed and are removed

This CL is part of the ongoing effort to eliminate the legacy IPC
system.

Change-Id: Icc8a679d0da0f58df99f9692ba2be5ebee42966b
Bug: 1007300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834091
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703241}