Ensure the text autosizer creates a root cluster when needed

LayoutTests/fast/text-autosizing/anonymous-block-crash.html

Index: LayoutTests/fast/text-autosizing/anonymous-block-crash.html
diff --git a/LayoutTests/fast/text-autosizing/anonymous-block-crash.html b/LayoutTests/fast/text-autosizing/anonymous-block-crash.html
new file mode 100644
index 0000000000000000000000000000000000000000..a5fb2bce93386b05ff6d07ad1c15dcee977c569f
--- /dev/null
+++ b/LayoutTests/fast/text-autosizing/anonymous-block-crash.html
@@ -0,0 +1,13 @@
+Test for crbug.com/521657: This test passes if it does not crash.

[skobes, 2015/08/24 17:47:57]

I think your -expected.txt should have this text in it?

[mstensho (USE GERRIT), 2015/08/24 17:51:49]

No, because the final result is a blank page. This text node will be gone.

[skobes, 2015/08/24 17:57:37]

Ah, right :)

+<script>
+if (window.testRunner)
+    testRunner.dumpAsText();
+internals.settings.setTextAutosizingEnabled(true);
+internals.settings.setTextAutosizingWindowSizeOverride(320, 480);
+
+document.documentElement.style.overflow = '-webkit-paged-x';
+onload = function() {
+    var forceLayout = document.body.offsetTop;
+    document.documentElement.parentNode.removeChild(document.documentElement);
+}
+</script>