Fix regression in the extension UI

This div needs min-height: 0 after crrev.com/c/1269235

R=rdevlin.cronin@chromium.org

Bug: 899177,899178
Change-Id: Ia000eb4f22b21247743986b81ae3190322d2c63e
Reviewed-on: https://chromium-review.googlesource.com/c/1306033Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603706}

chrome/browser/resources/md_extensions/item.html

@@ -76,6 +76,7 @@
       #main {
         display: flex;
         flex: 1;
+        min-height: 0;
         padding: 16px 20px;
       }