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}