[d8] move 'Synopsis' from flags.cc into d8.cc

This commit moves the d8 Synopsis/Usage string from flags.cc into d8.cc.

The motivation for this is that Commit
c8679386 ("[snapshot] enable mksnapshot
usage to be displayed") enabled a usage string to be passed into

means mksnapshot --help will first print it's own usage string followed
by d8's.

FlagList: :SetFlagsFromCommandLine and be printed, but
FlagList: :PrintHelp currently still prints the Synopsis for d8 which
Change-Id: I9c79b2cdc5570106e12096c0c53e640cf80742e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2299365Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68912}

src/d8/d8.cc

@@ -38,6 +38,7 @@
 #include "src/deoptimizer/deoptimizer.h"
 #include "src/diagnostics/basic-block-profiler.h"
 #include "src/execution/vm-state-inl.h"
+#include "src/flags/flags.h"
 #include "src/handles/maybe-handles.h"
 #include "src/init/v8.h"
 #include "src/interpreter/interpreter.h"
@@ -3383,7 +3384,16 @@ bool Shell::SetOptions(int argc, char* argv[]) {
     }
   }
 
-  v8::V8::SetFlagsFromCommandLine(&argc, argv, true);
+  const char* usage =
+      "Synopsis:\n"
+      "  shell [options] [--shell] [<file>...]\n"
+      "  d8 [options] [-e <string>] [--shell] [[--module] <file>...]\n\n"
+      "  -e        execute a string in V8\n"
+      "  --shell   run an interactive JavaScript shell\n"
+      "  --module  execute a file as a JavaScript module\n\n";
+  using HelpOptions = i::FlagList::HelpOptions;
+  i::FlagList::SetFlagsFromCommandLine(&argc, argv, true,
+                                       HelpOptions(HelpOptions::kExit, usage));
   options.mock_arraybuffer_allocator = i::FLAG_mock_arraybuffer_allocator;
   options.mock_arraybuffer_allocator_limit =
       i::FLAG_mock_arraybuffer_allocator_limit;

src/flags/flags.cc

@@ -568,21 +568,14 @@ void FlagList::PrintHelp() {
   CpuFeatures::PrintFeatures();
 
   StdoutStream os;
-  os << "Synopsis:\n"
-        "  shell [options] [--shell] [<file>...]\n"
-        "  d8 [options] [-e <string>] [--shell] [[--module] <file>...]\n\n"
-        "  -e        execute a string in V8\n"
-        "  --shell   run an interactive JavaScript shell\n"
-        "  --module  execute a file as a JavaScript module\n\n"
-        "Note: the --module option is implicitly enabled for *.mjs files.\n\n"
-        "The following syntax for options is accepted (both '-' and '--' are "
+  os << "The following syntax for options is accepted (both '-' and '--' are "
         "ok):\n"
         "  --flag        (bool flags only)\n"
         "  --no-flag     (bool flags only)\n"
         "  --flag=value  (non-bool flags only, no spaces around '=')\n"
         "  --flag value  (non-bool flags only)\n"
-        "  --            (captures all remaining args in JavaScript)\n\n"
-        "Options:\n";
+        "  --            (captures all remaining args in JavaScript)\n\n";
+  os << "Options:\n";
 
   for (const Flag& f : flags) {
     os << "  --";