[snapshot] fix check script_context_table

There maybe one more ScriptContext in script_context_table if mksnapshot
executed with embeded script.

R=yangguo@chromium.org

Bug: v8:7304
Change-Id: I6f431f76d4beae61a519c3acc3b856d1f7479740
Reviewed-on: https://chromium-review.googlesource.com/866630Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50637}

src/bootstrapper.cc

@@ -5555,10 +5555,6 @@ Genesis::Genesis(
 
   ConfigureUtilsObject(context_type);
 
-  // Check that the script context table is empty except for the 'this' binding.
-  // We do not need script contexts for native scripts.
-  DCHECK_EQ(1, native_context()->script_context_table()->used());
-
   native_context()->ResetErrorsThrown();
   result_ = native_context();
 }

src/runtime/runtime-scopes.cc

@@ -9,6 +9,7 @@
 #include "src/accessors.h"
 #include "src/arguments.h"
 #include "src/ast/scopes.h"
+#include "src/bootstrapper.h"
 #include "src/deoptimizer.h"
 #include "src/frames-inl.h"
 #include "src/isolate-inl.h"
@@ -726,6 +727,9 @@ RUNTIME_FUNCTION(Runtime_NewScriptContext) {
   Handle<JSFunction> closure(function->shared()->IsUserJavaScript()
                                  ? native_context->closure()
                                  : *function);
+
+  // We do not need script contexts here during bootstrap.
+  DCHECK(!isolate->bootstrapper()->IsActive());
   Handle<Context> result =
       isolate->factory()->NewScriptContext(closure, scope_info);