[turbofan] Add single --turbo flag.

src/v8.cc

Index: src/v8.cc
diff --git a/src/v8.cc b/src/v8.cc
index 4c3c023af62f687f272288d495368725f52a5fbf..49a104f0aa588553268b7003ffc72fed90e9d436 100644
--- a/src/v8.cc
+++ b/src/v8.cc
@@ -77,6 +77,11 @@ void V8::InitializeOncePerProcessImpl() {
     FLAG_max_semi_space_size = 1;
   }
 
+  if (FLAG_turbo && strcmp(FLAG_turbo_filter, "~~") == 0) {
+    const char* filter_flag = "--turbo-filter=*";
+    FlagList::SetFlagsFromString(filter_flag, StrLength(filter_flag));
+  }
+
   base::OS::Initialize(FLAG_random_seed, FLAG_hard_abort, FLAG_gc_fake_mmap);
 
   Isolate::InitializeOncePerProcess();