[regexp] Enable fallback to experimental engine by default

This CL enables the functionality that was added in d4febb6b by
flipping the corresponding feature flag.

Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Bug: v8:10765
Bug: v8:11021
Change-Id: Id061a274b016c71e6a4f7d7934a9c287d3124228
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2470568
Commit-Queue: Martin Bidlingmaier <mbid@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70510}

src/flags/flag-definitions.h

@@ -1492,7 +1492,7 @@ DEFINE_BOOL(enable_experimental_regexp_engine, false,
 DEFINE_BOOL(trace_experimental_regexp_engine, false,
             "trace execution of experimental regexp engine")
 
-DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, false,
+DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, true,
             "fall back to a breadth-first regexp engine on excessive "
             "backtracking")
 DEFINE_UINT(regexp_backtracks_before_fallback, 10000,