[regexp] Disable fallback to experimental engine by default

This CL disables the fallback to the experimental breadth-first regexp
engine which was enabled in 1e1f9ffc.

Bug: chromium:1157044
Change-Id: I669b18eddc15ea49aa58192102e719ae7f0364fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593250Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72001}

src/flags/flag-definitions.h

@@ -1506,7 +1506,7 @@ DEFINE_IMPLICATION(default_to_experimental_regexp_engine,
 DEFINE_BOOL(trace_experimental_regexp_engine, false,
             "trace execution of experimental regexp engine")
 
-DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, true,
+DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, false,
             "fall back to a breadth-first regexp engine on excessive "
             "backtracking")
 DEFINE_UINT(regexp_backtracks_before_fallback, 50000,