WebRequest: Restrict the blocking capabilities of the webRequest API.

In Manifest V3, we will strive to limit the blocking version of webRequest, potentially removing blocking options from most events (making them observational only).

The declarativeNetRequest API³ is an alternative to the webRequest API.  At its core, this API allows extensions to tell Chrome what to do with a given request, rather than have Chrome forward the request to the extension.
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This allows us to ensure efficiency since a) we have control over the algorithm determining the result and b) we can prevent or disable inefficient rules.  This is also better for user privacy, as the details of the network request are never exposed to the extension.

Chrome is deprecating the blocking capabilities of the webRequest API in Manifest V3, not the entire webRequest API (though blocking will still be available to enterprise deployments)

The first and IMO largest change is that Chrome now has support for dynamic modification of DNR rules ... DNR has two groups of rules: static rules declared in JSON files and dynamic rules set at runtime. ... We are planning to raise these values but we won't have updated numbers until we can run performance tests to find a good upper bound that will work across all supported devices.

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