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This makes FrameTokens consistent with WorkerTokens. A further CL will
migrate usage of base::UnguessableToken to FrameToken.

BUG=1096617

Change-Id: If29b4e50b17b4ead953b378339df214afd619ddd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314276
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792301}