[dict-proto] Add v8_dict_mode_prototypes build flag

This adds a flag behind which we want to do the work towards allowing
prototype objects to stay in dict/"slow" mode rather than switching
them back to fast mode

Bug: v8:7569

Change-Id: I3c963dea5d01be3c348810f40f8610fc2a488819
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450015Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Frank Emrich <emrich@google.com>
Cr-Commit-Position: refs/heads/master@{#70367}

BUILD.gn

@@ -282,6 +282,10 @@ declare_args() {
   # Experimental feature for collecting per-class zone memory stats.
   # Requires use_rtti = true
   v8_enable_precise_zone_stats = false
+
+  # Experimental feature for always keeping prototypes in dict/"slow" mode
+  # Sets -DV8_DICT_MODE_PROTOTYPES
+  v8_dict_mode_prototypes = false
 }
 
 # Derived defaults.
@@ -674,6 +678,9 @@ config("features") {
   if (v8_fuzzilli) {
     defines += [ "V8_FUZZILLI" ]
   }
+  if (v8_dict_mode_prototypes) {
+    defines += [ "V8_DICT_MODE_PROTOTYPES" ]
+  }
 }
 
 config("toolchain") {

src/common/globals.h

@@ -103,6 +103,13 @@ STATIC_ASSERT(V8_DEFAULT_STACK_SIZE_KB* KB +
 #define V8_DOUBLE_FIELDS_UNBOXING false
 #endif
 
+// Determine whether dict mode prototypes feature is enabled.
+#ifdef V8_DICT_MODE_PROTOTYPES
+#define V8_DICT_MODE_PROTOTYPES_BOOL true
+#else
+#define V8_DICT_MODE_PROTOTYPES_BOOL false
+#endif
+
 // Determine whether tagged pointers are 8 bytes (used in Torque layouts for
 // choosing where to insert padding).
 #if V8_TARGET_ARCH_64_BIT && !defined(V8_COMPRESS_POINTERS)