MD Extension: full subpage height.

Follow up of 598504 (missed committing a change that I made locally).

Bug: 741226
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I03848ef5dd9c9207262c2d48f491d678980e2b9c
Reviewed-on: https://chromium-review.googlesource.com/599221Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491854}

chrome/browser/resources/md_extensions/detail_view.html

@@ -28,6 +28,7 @@
       #main {
         background-color: white;
         margin: auto;
+        min-height: 100%;
         width: 640px;
       }