[Cronet] Rename reportFinished() to reportRequestFinished().

This is the Java-side followup to the rename request in the native API
made here [1].

[1] https://crrev.com/c/1496296/26/components/cronet/native/engine.h#71

Bug: 879208
Change-Id: I0eb0bcc8c7e7f1a2376c271e45f56b9f457c22f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574227
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652548}