Allows reading GC object stats data from an unpacked Chrome trace file.

No-try: true
Bug: v8:7266
Change-Id: I1b851a5b9c5937bd80ae08129d3daee8b4397500
Reviewed-on: https://chromium-review.googlesource.com/868211Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50622}