[parser] Fix template escapes in preparser

BUG=v8:6100

Change-Id: Ib8729b2688bbaf6fb397737ccf1b1c086698ab93
Reviewed-on: https://chromium-review.googlesource.com/455876
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43822}

src/parsing/parser.h

@@ -314,6 +314,7 @@ class V8_EXPORT_PRIVATE Parser : public NON_EXPORTED_BASE(ParserBase<Parser>) {
       SET_ALLOW(harmony_object_rest_spread);
       SET_ALLOW(harmony_dynamic_import);
       SET_ALLOW(harmony_async_iteration);
+      SET_ALLOW(harmony_template_escapes);
 #undef SET_ALLOW
     }
     return reusable_preparser_;

src/parsing/preparser.cc

@@ -108,7 +108,7 @@ PreParser::PreParseResult PreParser::PreParseProgram(bool is_module) {
   if (stack_overflow()) return kPreParseStackOverflow;
   if (!ok) {
     ReportUnexpectedToken(scanner()->current_token());
-  } else if (is_strict(this->scope()->language_mode())) {
+  } else if (is_strict(language_mode())) {
     CheckStrictOctalLiteral(start_position, scanner()->location().end_pos, &ok);
   }
   return kPreParseSuccess;