CRX Telemetry - off-store data improvements

- Switches order in FileProcessor to check manifest.json validity first
- Add histogram for missed files over max file to process limit
- Add histogram for valid extensions seen
- Update DumpReportForTest() method to include off-store data

Bug: 1384755
Change-Id: I13c8cb7360c2cb2e89eda17934d6d7e0ba8b1e4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4343292


Reviewed-by: Anunoy Ghosh <anunoy@chromium.org>
Reviewed-by: Zack Han <zackhan@chromium.org>
Commit-Queue: Richard Chen <richche@google.com>
Reviewed-by: thefrog <thefrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118896}