Default to the same Cargo features in all build commands

Fixes #8308 and #7327.

python/servo/command_base.py

@@ -415,6 +415,13 @@ class CommandBase(object):
     def servo_crate(self):
         return path.join(self.context.topdir, "components", "servo")
 
+    def servo_features(self):
+        """Return a list of optional features to enable for the Servo crate"""
+        features = []
+        if self.config["build"]["debug-mozjs"]:
+            features += ["script/debugmozjs"]
+        return features
+
     def android_support_dir(self):
         return path.join(self.context.topdir, "support", "android")