Private browsing session can be detected through `webkitTemporaryStorage`

[antonok-edm]

The following code returns true when executed in a private browsing session ("incognito mode"):

await new Promise(resolve => navigator.webkitTemporaryStorage.queryUsageAndQuota(function(_, quota) {
    const quotaInMib = Math.round(quota / (1024 * 1024));
    const quotaLimitInMib = Math.round(window.performance.memory.jsHeapSizeLimit / (1024 * 1024)) * 2;
    resolve(quotaInMib < quotaLimitInMib);
}))

^{adapted from Joe12387/detectIncognito}

The problematic API here is navigator.webkitTemporaryStorage. The newer, standardized version of that API, navigator.storage, was patched because it allowed sites to detect private tabs, and also because it provided a fingerprintable persistent identifier. We should give the same treatment to this older non-standard API.

[antonok-edm]

Note the following Chromium code for QuotaManagerImpl::ContinueIncognitoGetStorageCapacity:

https://source.chromium.org/chromium/chromium/src/+/main:storage/browser/quota/quota_manager_impl.cc;l=2808?q=quota_manager_impl.cc&ss=chromium