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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ports/libsimpleservo/Cargo.toml

@@ -42,3 +42,5 @@ debugmozjs = ["libservo/debugmozjs"]
 unstable = ["libservo/unstable"]
 googlevr = ["libservo/googlevr"]
 oculusvr = ["libservo/oculusvr"]
+webgl_backtrace = ["libservo/webgl_backtrace"]
+js_backtrace = ["libservo/js_backtrace"]

ports/servo/Cargo.toml

@@ -30,6 +30,8 @@ webdriver = ["libservo/webdriver"]
 energy-profiling = ["libservo/energy-profiling"]
 debugmozjs = ["libservo/debugmozjs"]
 unstable = ["libservo/unstable"]
+webgl_backtrace = ["libservo/webgl_backtrace"]
+js_backtrace = ["libservo/js_backtrace"]
 
 [target.'cfg(not(target_os = "android"))'.dependencies]
 backtrace = "0.3"