MD Extensions: fix subpage height and scroll issues.

The subpage's main container is too short and contents are leaking out of it. This CL makes it such that the main-container spans full height and puts the overflow scrolling on the parent detail-view element.

Bug: 741226
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic0fe061435bc82544058533126e5eba75be8f357
Reviewed-on: https://chromium-review.googlesource.com/598504Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491575}

chrome/browser/resources/md_extensions/detail_view.html

@@ -20,13 +20,14 @@
     <style include=
         "iron-flex cr-hidden-style cr-icons action-link paper-toggle-style">
       :host {
-        display: flex;
-        justify-content: center;
+        display: block;
+        height: 100%;
+        overflow: auto;
       }
 
       #main {
         background-color: white;
-        height: 100%;
+        margin: auto;
         width: 640px;
       }