Currently objects that belong to the untracked contexts (i.e. contexts
for which measurement was not requested) are accounted in the shared
context. This CL introduces a dummy kOtherContext and attributes such
objects to that context.

Bug: chromium:973627
Change-Id: I9801ab317d95b944336b79a5e17721511d4897c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2025370Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66019}