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Like workers, this defines a collection of tokens for identifying
distinct Worklets. This also updates ExecutionContextToken so that it is
able to name all possible ExecutionContext types. All of these token
types will be used by the performance.measureMemory API.

BUG=1085129

Change-Id: I8ad41aeac689278f1e47502c98855dd4aec41b3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354451Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798082}