[es7] Add --harmony-explicit-tailcalls flag.

BUG=v8:4915
LOG=N

Review URL: https://codereview.chromium.org/1891633003

Cr-Commit-Position: refs/heads/master@{#35484}

src/bootstrapper.cc

@@ -2405,6 +2405,7 @@ EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_regexp_property)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_function_name)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_function_sent)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(promise_extra)
+EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_explicit_tailcalls)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_tailcalls)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_instanceof)
 EMPTY_INITIALIZE_GLOBAL_FOR_FEATURE(harmony_restrictive_declarations)
@@ -2948,6 +2949,7 @@ bool Genesis::InstallExperimentalNatives() {
   static const char* harmony_iterator_close_natives[] = {nullptr};
   static const char* harmony_species_natives[] = {"native harmony-species.js",
                                                   nullptr};
+  static const char* harmony_explicit_tailcalls_natives[] = {nullptr};
   static const char* harmony_tailcalls_natives[] = {nullptr};
   static const char* harmony_unicode_regexps_natives[] = {
       "native harmony-unicode-regexps.js", nullptr};

src/flag-definitions.h

@@ -194,6 +194,7 @@ DEFINE_IMPLICATION(es_staging, move_object_start)
 
 // Features that are still work in progress (behind individual flags).
 #define HARMONY_INPROGRESS(V)                                           \
+  V(harmony_explicit_tailcalls, "harmony explicit tail calls")          \
   V(harmony_object_observe, "harmony Object.observe")                   \
   V(harmony_function_sent, "harmony function.sent")                     \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")             \