MD Extensions: Fix corner-case where available width is less than padding.

Bug: 823208
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I93a95f26e5910fe3fa98f6ad43c0d629bbf4f587
Reviewed-on: https://chromium-review.googlesource.com/969095Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544195}

chrome/browser/resources/md_extensions/item_list.html

@@ -10,6 +10,12 @@
 <dom-module id="extensions-item-list">
   <template>
     <style include="cr-shared-style">
+      :host {
+        /* Address corner-case where #container's padding is larger than max
+           available width. */
+        overflow: auto;
+      }
+
       .items-container,
       extensions-item {
         --extensions-card-width: 400px;