[tools/perf] Provide reason for disabling testValidDump

TBR=crouleau@chromium.org

Bug: 971998
Change-Id: Ifc468d1bcecd09756c15b392d999f8f78499f3b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649500
Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667079}