Prevent onSync infinite loops

[philipwalton]

R: @jeffposnick

Fixes #1937

This PR updates the background sync replay logic in the following ways to help prevent infinite loops when adding new requests to the queue during a sync event replay:

• Adding a request to the queue only registers a sync tag if a sync event is not currently in progress.
• At the end of a successful sync event replay (i.e. the waitUntil promise did not reject) a sync tag is registered if new requests have been added to the queue.
• At the end of a failed sync event replay (i.e. the waitUntil promise rejected) no sync event is registered because the browser will automatically retry later (unless the event.lastChance property is true, in which case a new sync tag is registered

The PR also updates some of the workbox-google-analytics behavior to handle these new changes.

[workbox-pr-bot]

PR-Bot Size Plugin

Changed File Sizes

File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.21 KB	3.38 KB	+5%	1.46 KB
packages/workbox-google-analytics/build/workbox-offline-ga.prod.js	1.89 KB	1.89 KB	-0%	897 B

New Files

No new files have been added.

All File Sizes

View Table

File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.21 KB	3.38 KB	+5%	1.46 KB
packages/workbox-broadcast-update/build/workbox-broadcast-update.prod.js	1.86 KB	1.86 KB	0%	933 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/index.js	3.64 KB	3.64 KB	0%	1.36 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	579 B	579 B	0%	344 B
packages/workbox-cli/build/app.js	5.58 KB	5.58 KB	0%	1.98 KB
packages/workbox-cli/build/bin.js	1.16 KB	1.16 KB	0%	580 B
packages/workbox-core/build/workbox-core.prod.js	5.34 KB	5.34 KB	0%	2.30 KB
packages/workbox-expiration/build/workbox-expiration.prod.js	2.83 KB	2.83 KB	0%	1.24 KB
packages/workbox-google-analytics/build/workbox-offline-ga.prod.js	1.89 KB	1.89 KB	-0%	897 B
packages/workbox-navigation-preload/build/workbox-navigation-preload.prod.js	652 B	652 B	0%	317 B
packages/workbox-precaching/build/workbox-precaching.prod.js	4.25 KB	4.25 KB	0%	1.71 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.51 KB	1.51 KB	0%	758 B
packages/workbox-routing/build/workbox-routing.prod.js	3.38 KB	3.38 KB	0%	1.47 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	4.86 KB	4.86 KB	0%	1.19 KB
packages/workbox-streams/build/workbox-streams.prod.js	1.38 KB	1.38 KB	0%	677 B
packages/workbox-sw/build/workbox-sw.js	1.33 KB	1.33 KB	0%	741 B
packages/workbox-webpack-plugin/build/generate-sw.js	5.29 KB	5.29 KB	0%	1.84 KB
packages/workbox-webpack-plugin/build/index.js	349 B	349 B	0%	255 B
packages/workbox-webpack-plugin/build/inject-manifest.js	7.22 KB	7.22 KB	0%	2.48 KB
packages/workbox-window/build/workbox-window.dev.umd.js	30.52 KB	30.52 KB	0%	8.07 KB
packages/workbox-window/build/workbox-window.prod.umd.js	4.66 KB	4.66 KB	0%	1.81 KB

Workbox Aggregate Size Plugin

8.77KB gzip'ed (58% of limit)
22.04KB uncompressed

[coveralls]

Coverage increased (+0.07%) to 82.736% when pulling 73f8006 on bg-sync-loop into 5e2c4a1 on master.

[jeffposnick]

Thanks for wading through the edge cases!