cronet: Add compatibility with MigratingDeframer

The tests for cronet use mocks to assume specific behavior about the stream, so were not trivial change to support MigratingDeframer. We should improve the tests so they can work with MigratingDeframer (or not), or decide we don't actually need MigratingDeframer for cronet.

Start with the cronet tests and their mocks, then inspect how they assume specific stream behavior and compare those assumptions with MigratingDeframer. Done means the tests can support the intended deframer behavior, or the project has clearly recorded that cronet does not need MigratingDeframer.

Vom Indexierungsmodell aus dem Issue-Text verfasst.