cppgc: young-gen: Enable cppgc_enable_young_generation by default

Oilpan Young Generation is now controlled by the runtime flag
--cppgc-young-generation.

Bug: chromium:1029379
Change-Id: I9ded9637f43a2f86993cff898cd7f272a051ae3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616728Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80312}

BUILD.gn

@@ -790,6 +790,9 @@ if (cppgc_enable_object_names) {
 }
 if (cppgc_enable_caged_heap) {
   enabled_external_cppgc_defines += [ "CPPGC_CAGED_HEAP" ]
+
+  # Always enable young generation compile time flag if caged heap is enabled.
+  cppgc_enable_young_generation = true
 }
 if (cppgc_enable_young_generation) {
   enabled_external_cppgc_defines += [ "CPPGC_YOUNG_GENERATION" ]