Bug 903212 - Add a GC when popping a script entry point. r=mrbkap

I think that this is more or less isomorphic with the MaybeGC we do in
ScriptEvaluated in the cases that matter. And doing it in a spec-defined
place is helpful in getting rid of spec-nonsensical machinery.

js/xpconnect/src/nsCxPusher.cpp

@@ -146,6 +146,15 @@ AutoCxPusher::AutoCxPusher(JSContext* cx, bool allowNull) : mScriptIsRunning(fal
 NS_EXPORT
 AutoCxPusher::~AutoCxPusher()
 {
+  // GC when we pop a script entry point. This is a useful heuristic that helps
+  // us out on certain (flawed) benchmarks like sunspider, because it lets us
+  // avoid GCing during the timing loop.
+  //
+  // NB: We need to take care to only do this if we're in a compartment,
+  // otherwise JS_MaybeGC will segfault.
+  if (mScx && !mAutoCompartment.empty())
+    JS_MaybeGC(nsXPConnect::XPConnect()->GetCurrentJSContext());
+
   // Leave the compartment and request before popping.
   mAutoCompartment.destroyIfConstructed();
   mAutoRequest.destroyIfConstructed();