Make enabling of CSA verifier a build-time flag

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 7711227b11ae7680e71ea0e17fd03201028e01ea..573be791564493c7e99ac7123900cbfb0db5cd95 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -55,6 +55,9 @@ declare_args() {
   # Enable slow dchecks.
   v8_enable_slow_dchecks = false
 
+  # Enable code-generation-time checking of types in the CodeStubAssembler.
+  v8_enable_verify_csa = false
+
   # Interpreted regexp engine exists as platform-independent alternative
   # based where the regular expression is compiled to a bytecode.
   v8_interpreted_regexp = false
@@ -402,6 +405,10 @@ config("toolchain") {
     defines += [ "DEBUG" ]
   }
 
+  if (v8_enable_verify_csa) {
+    defines += [ "ENABLE_VERIFY_CSA" ]
+  }
+
   if (v8_no_inline) {
     cflags += [
       "-fno-inline-functions",