Initialize child framesets when they become part of the parent frameset grid.

third_party/WebKit/LayoutTests/fast/frames/expand-grid-with-zero-size-child-frameset-crash.html

Index: third_party/WebKit/LayoutTests/fast/frames/expand-grid-with-zero-size-child-frameset-crash.html
diff --git a/third_party/WebKit/LayoutTests/fast/frames/expand-grid-with-zero-size-child-frameset-crash.html b/third_party/WebKit/LayoutTests/fast/frames/expand-grid-with-zero-size-child-frameset-crash.html
new file mode 100644
index 0000000000000000000000000000000000000000..f1adebe4d27a0e08fa02623afa9e7ceafd619712
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/fast/frames/expand-grid-with-zero-size-child-frameset-crash.html
@@ -0,0 +1,18 @@
+<script>
+    if (window.testRunner)
+        testRunner.dumpAsText();
+    onload = ()=> {
+        // Force layout first.
+        document.documentElement.offsetTop;
+
+        // Then expand the root frameset grid from 2x1 to 2x2.
+        document.getElementById("rootSet").setAttribute("rows", "*, 0");
+    }
+</script>
+<frameset id="rootSet" cols="0, *">
+    <frame id="firstChild"></frame>
+    <frame src="data:text/html,PASS if no crash or assertion failure."></frame>
+    <frameset>
+         <frame src="data:text/html,3"></frame>
+     </frameset>
+</frameset>