Add tokens to WorkletGlobalScopes.

Similar to workers, this adds tokens to WorkletGlobalScopes so they can
be uniquely identified in a renderer process. Will be used by
performance.measureMemory implementation.

BUG=1085129

Change-Id: Ia7f194f1864639e19d9ddb9ee2d70cc6e00ba183
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351286


Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798123}