This is a reland of d7b67ce2

Original change's description:
> [Context] Add a bit flag to indicate if extension might exist
> 
> Checking the bit flag instead of comparing pointers should improve performance.
> This will also allow us to remove the extension slot in Context and save memory.
> 
> Bug: v8:9744
> Change-Id: I7ab9feeadfb934955798d877d13bc0e1d78a191c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1814918
> Commit-Queue: Victor Gomes <victorgomes@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63906}

Bug: v8:9744
Change-Id: Ic4725ad5730a8f8fff6288d6af2205c230aff79d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1815256Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@google.com>
Cr-Commit-Position: refs/heads/master@{#63993}