Deflake network-requests test by only counting headers

This test fails sometimes because the set of headers changes in a run:

```
Unexpected Diff (+got, -want):
+ResourceReceiveResponse headers: Accept-Ranges, Content-Length, Content-Type, Date, ETag, Last-Modified, Server
-ResourceReceiveResponse headers: Accept-Ranges, Connection, Content-Length, Content-Type, Date, ETag, Keep-Alive, Last-Modified, Server
```

So this CL updates the test to just check that the trace event included
headers.

Bug: 40268741
Change-Id: I6ddb8f225be8d82c3264fecbef046ddd7ace82ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5773057
Auto-Submit: Jack Franklin <jacktfranklin@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1338970}