Add a constraint on how many requests can be outstanding for any given render process (browser-side).
Once the constraint is reached, subsequent requests will fail with net::ERR_INSUFFICIENT_RESOURCES.
The bound is defined as "25 MB", which represents the amount of private bytes we expect the pending requests to consume in the browser. This number translates into around 6000 typical requests.
Note that the upload data of a request is not currently considered part of the request's in-memory cost -- more data is needed on the average/maximum upload sizes of users before deciding what a compatible limit is.