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Only allow Periodic Background Sync registration if the requesting
origin has a main frame host present.

Add browsertests to verify this behavior.

Corresponding PR against the spec: https://github.com/WICG/BackgroundSync/pull/160

Bug: 1029712
Change-Id: I1142578edbde62f3f27ae95b9ca602855df3543b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942433
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720483}