Handle nested theads correctly when paginated

When paginated, repeat nested theads at the top of each page - replicating the
way they look on the first page bar any spacing between them applied by content.

We don't try to replicate any cell padding in the nested repeating theads, that
would be a lot of work for not much gain.

BUG=620456

Review-Url: https://codereview.chromium.org/2079603002
Cr-Commit-Position: refs/heads/master@{#402943}
diff --git a/third_party/WebKit/LayoutTests/fragmentation/single-line-cells-nested-repeating-thead-4.html b/third_party/WebKit/LayoutTests/fragmentation/single-line-cells-nested-repeating-thead-4.html
new file mode 100644
index 0000000..2abdbc4
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/fragmentation/single-line-cells-nested-repeating-thead-4.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<style>
+table {
+    border-collapse: collapse;
+}
+td, th {
+}
+thead, tr {
+    break-inside: avoid;
+}
+</style>
+<p>crbug.com/620456: A nested table header group repeats on each page.</p>
+<div style="-webkit-columns:3; line-height: 20px; column-fill: auto; height:190px;">
+    <table id="top">
+        <thead id="top-thead">
+            <tr>
+                <th>Col 1</th>
+            </tr>
+        </thead>
+        <tr><td>
+                <br>
+                    <table id="next">
+                        <thead id="next-thead">
+                            <tr>
+                                <th>Col 1</th>
+                                <th>Col 2</th>
+                            </tr>
+                        </thead>
+                        <tbody>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        <tr><td>Te</td><td>xt</td></tr>
+                        </tbody>
+                    </table>
+        </td></tr>
+    </table>
+</div>
+