This makes it possible to plumb string representation of stack trace id
across various channels, e.g. for network requests.

Drive-by: extracted class V8DebuggerId, which encapsulates operations
with pair<int64_t, int64_t>.

Bug: chromium:988842
Change-Id: I348c91390a85bf07c746d1b1c4a7775f44c7d769
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1725193
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63900}