This will be used in CSA to check if any promisehook is set.

-- Adds a is_promisehook_enabled_ field to the isolate and helper methods.
-- Adds this field to the ExternalReference table.
-- Adds a helper method to access this from CSA

Note -- this patch doesn't actually add the ability to attach the hook
yet.

BUG=v8:4643

Review-Url: https://codereview.chromium.org/2566483002
Cr-Commit-Position: refs/heads/master@{#41607}