Auto merge of #21825 - jdm:backtrace-helpers, r=Manishearth

Optional backtraces for JS errors and WebGL errors

This adds two new build-time features to enable useful debugging tools when investigating why JS and WebGL content isn't working. They're optional because they're quite heavyweight.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they're optional developer features.

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python/servo/command_base.py

@@ -288,6 +288,8 @@ class CommandBase(object):
         self.config["build"].setdefault("rustflags", "")
         self.config["build"].setdefault("incremental", None)
         self.config["build"].setdefault("thinlto", False)
+        self.config["build"].setdefault("webgl-backtrace", False)
+        self.config["build"].setdefault("dom-backtrace", False)
 
         self.config.setdefault("android", {})
         self.config["android"].setdefault("sdk", "")