The existing legacy performance.memory API accounts external string
and array buffer backing store bytes. This CL adds per-context tracking
of external bytes

Bug: chromium:973627
Change-Id: I2b308dc540454e7b0b66406b83a18bf8f8d55d8e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2025369Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66018}