Bug 927349 part 17 - Don't count native reflows in test_popupreflows.xul; r=markh

test_popupreflows.xul currently expects to get 1 reflow when a popup is opened.
This is triggered by JS code. An incremental layout may be triggered by native
code but the test ignores that for unclear reasons ("Because we've simply
cargo-culted this test from browser_tabopen_reflows.js!").

In part 16 on this patch series, we force interruptible reflows to become
full reflows when we have pending animations which can cause this test to
fail since it will see 2 reflows.

The above quoted browser_tabopen_reflows.js specifically ignores reflows
triggered by native code so we adjust this test to do likewise.

toolkit/content/tests/widgets/test_popupreflows.xul

@@ -26,6 +26,13 @@ let panel, anchor, arrow;
 let observer = {
   reflows: [],
   reflow: function (start, end) {
+    // Ignore reflows triggered by native code
+    // (Reflows from native code only have an empty stack after the first frame)
+    var path = (new Error().stack).split("\n").slice(1).join("");
+    if (path === "") {
+      return;
+    }
+
     this.reflows.push(new Error().stack);
   },