Sprinkle tracing events around ProxyChangeListener

This code can keep the main thread and the Cronet init thread busy, but
traces currently won't show anything at all; this CL fixes that.

The impact on trace sizes/performance is negligible because proxy change
events are very rare.

Change-Id: I2a1ac0d4a5db9eefa65be669d5783e6fec597bb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6633532
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Auto-Submit: Etienne Dechamps <edechamps@google.com>
Reviewed-by: Stefano Duo <stefanoduo@google.com>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1473850}