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This defines a handful of MultiToken<> variants (and their mojom traits)
that will be used in renderer code, as well as in renderer to browser IPC.

BUG=1085129, 1096617

Change-Id: I548ecfe2824d2a8e78c0af8a197abebf2ab6fc41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337104
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795182}