Extensions: fix scroll

Bug: 837177
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I84600d1e162db6994e56c99297645393aeab43d8
Reviewed-on: https://chromium-review.googlesource.com/1091652
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565524}

chrome/browser/resources/md_extensions/item.html

@@ -67,7 +67,6 @@
         height: 160px;
         /* Duration matches --drawer-transition from toolbar.html. */
         transition: height 300ms cubic-bezier(.25, .1, .25, 1);
-        width: var(--extensions-card-width, 400px);
       }
 
       #card.dev-mode {

chrome/browser/resources/md_extensions/item_list.html

@@ -11,17 +11,18 @@
   <template>
     <style include="cr-shared-style">
       .items-container,
-      extensions-item {
+      #content-wrapper {
         --extensions-card-width: 400px;
       }
 
       #container {
         box-sizing: border-box;
         height: 100%;
-        overflow: overlay;
+        overflow: auto;
       }
 
       #content-wrapper {
+        min-width: var(--extensions-card-width);
         padding: 24px 60px 64px;
       }