Bug 1732362 part 2 - Initialize JS engine after parsing command line options. r=jonco

This allows us to have shell flags that affect JS_Init.

Depends on D136722

Differential Revision: https://phabricator.services.mozilla.com/D136723

js/src/shell/js.cpp

@@ -11896,20 +11896,6 @@ int main(int argc, char** argv) {
   SetOutputFile("JS_STDOUT", &rcStdout, &gOutFile);
   SetOutputFile("JS_STDERR", &rcStderr, &gErrFile);
 
-  // Start the engine.
-  if (const char* message = JS_InitWithFailureDiagnostic()) {
-    fprintf(gErrFile->fp, "JS_Init failed: %s\n", message);
-    return 1;
-  }
-
-  // `selfHostedXDRBuffer` contains XDR buffer of the self-hosted JS.
-  // A part of it is borrowed by ImmutableScriptData of the self-hosted scripts.
-  //
-  // This buffer's should outlive JS_Shutdown.
-  Maybe<FileContents> selfHostedXDRBuffer;
-
-  auto shutdownEngine = MakeScopeExit([] { JS_ShutDown(); });
-
   OptionParser op("Usage: {progname} [options] [[script] scriptArgs*]");
 
   op.setDescription(
@@ -12359,6 +12345,20 @@ int main(int argc, char** argv) {
     return EXIT_SUCCESS;
   }
 
+  // Start the engine.
+  if (const char* message = JS_InitWithFailureDiagnostic()) {
+    fprintf(gErrFile->fp, "JS_Init failed: %s\n", message);
+    return 1;
+  }
+
+  // `selfHostedXDRBuffer` contains XDR buffer of the self-hosted JS.
+  // A part of it is borrowed by ImmutableScriptData of the self-hosted scripts.
+  //
+  // This buffer's should outlive JS_Shutdown.
+  Maybe<FileContents> selfHostedXDRBuffer;
+
+  auto shutdownEngine = MakeScopeExit([] { JS_ShutDown(); });
+
   // Record aggregated telemetry data on disk. Do this as early as possible such
   // that the telemetry is recording both before starting the context and after
   // closing it.