Extensions WebUI: when window is very narrow such that the dev drawer

buttons wrap, show scroll

Bug: 905991
Change-Id: I63668926091cb84a9ba11c2c4a8310f54883ccbf
Reviewed-on: https://chromium-review.googlesource.com/c/1347200
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610392}

chrome/browser/resources/md_extensions/toolbar.html

@@ -53,7 +53,8 @@
         border-bottom: 1px solid var(--google-grey-300);
         box-sizing: border-box;
         height: 0;
-        overflow: hidden;
+        overflow-x: hidden;
+        overflow-y: auto;
         position: relative;
         transition: height var(--drawer-transition);
       }