Enable --always-use-string-forwarding-table behind --future

We temporarily enable the StringForwardingTable for all ThinString
transitions behind --future to get coverage and performance numbers from
future bots.

Bug: v8:12007
Change-Id: Ia59bd6c40df2fd413867f6fc2aa7f69d099b5377
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3637799Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80516}

src/flags/flag-definitions.h

@@ -757,6 +757,10 @@ DEFINE_IMPLICATION(harmony_struct, shared_string_table)
 DEFINE_BOOL(
     always_use_string_forwarding_table, false,
     "use string forwarding table instead of thin strings for all strings")
+// TOOD(pthier): The code behind this flag is not going to ship.
+// We enable it behind --future to get performance numbers and coverage from
+// bots.
+DEFINE_WEAK_IMPLICATION(future, always_use_string_forwarding_table)
 
 DEFINE_BOOL(write_code_using_rwx, true,
             "flip permissions to rwx to write page instead of rw")