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This lives under the Service Worker section of the Application tab in
Devtools and allows dispatching a periodicsync event with a given tag
so that developers can test how they respond to it.

Screenshots here:
https://bugs.chromium.org/p/chromium/issues/detail?id=961238#c4

Bug: 961238
Change-Id: I2f5fe2c41ad9d2c50db99c44d873d8cc61964913
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725893Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Auto-Submit: Mugdha Lakhani <nator@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685540}