allocator: add use_experimental_allocator_shim build flag

base/allocator/BUILD.gn

Index: base/allocator/BUILD.gn
diff --git a/base/allocator/BUILD.gn b/base/allocator/BUILD.gn
index 5270a91c491553f10d030301188ff47eb9a9068a..0049800cb8620724cc14d2985aace93d85c195a7 100644
--- a/base/allocator/BUILD.gn
+++ b/base/allocator/BUILD.gn
@@ -47,13 +47,17 @@ config("allocator_shim_define") {
 }
 
 config("tcmalloc_flags") {
+  defines = []
   if (enable_debugallocation) {
-    defines = [
+    defines += [
       # Use debugallocation for Debug builds to catch problems early
       # and cleanly, http://crbug.com/30715 .
       "TCMALLOC_FOR_DEBUGALLOCATION",
     ]
   }
+  if (use_experimental_allocator_shim) {
+    defines += [ "TCMALLOC_DONT_REPLACE_SYSTEM_ALLOC" ]
+  }
   if (is_clang) {
     cflags = [
       # tcmalloc initializes some fields in the wrong order.