[date] Use v8_flags for accessing flag values

Avoid the deprecated FLAG_* syntax, access flag values via the
{v8_flags} struct instead.

R=ishell@chromium.org

Bug: v8:12887
Change-Id: I2ef25bc50fdf12f0149f2cdfce7102f2cc0f25d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899196Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83261}

src/date/date.cc

@@ -53,7 +53,7 @@ void DateCache::ResetDateCache(
   after_ = &dst_[1];
   ymd_valid_ = false;
 #ifdef V8_INTL_SUPPORT
-  if (!FLAG_icu_timezone_data) {
+  if (!v8_flags.icu_timezone_data) {
 #endif
     local_offset_ms_ = kInvalidLocalOffsetInMs;
 #ifdef V8_INTL_SUPPORT
@@ -215,7 +215,7 @@ void DateCache::BreakDownTime(int64_t time_ms, int* year, int* month, int* day,
 int DateCache::GetLocalOffsetFromOS(int64_t time_ms, bool is_utc) {
   double offset;
 #ifdef V8_INTL_SUPPORT
-  if (FLAG_icu_timezone_data) {
+  if (v8_flags.icu_timezone_data) {
     offset = tz_cache_->LocalTimeOffset(static_cast<double>(time_ms), is_utc);
   } else {
 #endif