[Extensions WebIDL] Correct naming on callback members with returns

These were previously taking the 'name' value for the 'returns' object
from the name of the 'callback' used for their type. This CL corrects
this to actually use the member name instead and adds a test case for
it.

Fixed: 491157753
Change-Id: Ia2c2d3d204095e38c4caa64d8dd6e62d10e701df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7649083
Commit-Queue: Tim <tjudkins@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1597299}