[DevTools] Extend DevTools.NetworkPanelResponsePreviewOpened histogram.

This histogram collects usage on a per mime-type basis, which we don't
get via Visual Logging. We are still monitoring the DevToolsMediaType
histograms (also in the Sources panel) to ensure that we don't miss
anything important here.

Fixed: 379691503
Change-Id: I86b0f34f6469aa03aaf34c3f8ad394efa40f5bfe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6037936
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1385627}