[beacon-api] Reformat PendingBeacon-related web_tests

[chromium-wpt-export-bot]

• Switch from .html to .window.js as suggested in previous review.
• Switched from FlagSpecific tests to Virtual tests as suggested in 2.
• Moved everything to external/wpt/ and make them .tentative as this is currently Chromium-only feature.

Bug: 1293679, 1238155 Change-Id: Ia250ec09b5157a2f105d4ca4f5e96b4182c6c3cf

Reviewed-on: https://chromium-review.googlesource.com/3817678 WPT-Export-Revision: 847aa9f3b3877337526eda28ed0384238b8c9836