Only enable embedded builtins in snapshot builds

Currently, nosnap builds do not run mksnapshot and thus do not have a
chance to generate the embedded blob. In theory we could support this,
but let's just ensure we don't crash for now.

Bug: v8:6666,v8:7835
Change-Id: I7d3b1f772b296ae3bcaed1604e0d1e44834da1c0
Reviewed-on: https://chromium-review.googlesource.com/1092491Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53600}

BUILD.gn

@@ -357,7 +357,7 @@ config("features") {
   if (v8_check_microtasks_scopes_consistency) {
     defines += [ "V8_CHECK_MICROTASKS_SCOPES_CONSISTENCY" ]
   }
-  if (v8_enable_embedded_builtins) {
+  if (v8_enable_embedded_builtins && v8_use_snapshot) {
     defines += [ "V8_EMBEDDED_BUILTINS" ]
   }
   if (v8_use_multi_snapshots) {