PSA: Enabling partitioned 3rd party storage when “Block 3rd Party Cookies” setting is enabled

[Mike Taylor]

Howdy blink-dev,

In https://chromium-review.googlesource.com/c/chromium/src/+/4899223 we intend to enable the 3rd party storage partitioning feature when 3rd party cookies (3PCs) are disabled, given that 3rd party storage partitioning is enabled for 100% of M115 and above.

Previously, when a user blocked 3PCs, storage in 3rd party contexts was also blocked. Given that partitioned storage does not contribute to cross site tracking, it is desirable to allow this storage when 3PCs are blocked - especially as we move towards deprecating 3PCs. Note that user specified allow/block cookie settings will continue to work and a page with a user block cookie setting will also block all storage.

This follows closely in the footsteps of CHIPS, for which we already aligned on the acceptable privacy implications as well as the required UI updates. We have now landed the same UI treatment for partitioned storage as well, to avoid unpleasant surprises.

This change will ride the M119 train, so we should be able to observe any compatibility issues before it reaches stable (but we don’t anticipate any new breakage beyond what we've seen by shipping partitioned storage to stable).

thanks, Mike