[sandbox] Use v8_flags for accessing flag values

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

R=saelo@chromium.org

Bug: v8:12887
Change-Id: I7e41e1952958936c32fec501b8348fac0538cd71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899269Reviewed-by: Samuel Groß <saelo@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83265}

src/sandbox/sandbox.cc

@@ -215,8 +215,8 @@ bool Sandbox::InitializeAsPartiallyReservedSandbox(v8::VirtualAddressSpace* vas,
   // ourselves, and so are potentially better positioned to determine a good
   // base address for the sandbox than the embedder.
   base::RandomNumberGenerator rng;
-  if (FLAG_random_seed != 0) {
-    rng.SetSeed(FLAG_random_seed);
+  if (v8_flags.random_seed != 0) {
+    rng.SetSeed(v8_flags.random_seed);
   }
 
   // We try to ensure that base + size is still (mostly) within the process'