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This is a reland of 7c390807

The fix for the issue that caused the revert is here:
https://chromium-review.googlesource.com/c/chromium/src/+/1835164

Original change's description:
> [PM] Add SharedWorkerWatcher
>
> This class observes running shared worker instances, keeps track of
> the connection to their client frames and manages the lifetime of
> worker nodes in the Performance Manager graph.
>
> Bug: 993029
> Change-Id: I2e0c7cd9f194a06405293c22fd543a14ff3c6cbc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1757283
> Commit-Queue: Patrick Monette <pmonette@chromium.org>
> Reviewed-by: Matt Falkenhagen <falken@chromium.org>
> Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#701445}

Bug: 993029
Change-Id: Ib77ea36b9cc71ec83ccbe28173fc6053e4a497f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829796
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704055}