Extensions page: fix scrollbar regression when toggling dev mode

R=dpapad@chromium.org
BUG=930042

Change-Id: Ic20ceb781e363b033a56fbd541dabecd52f2158f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497541Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637597}

chrome/browser/resources/md_extensions/extensions.html

@@ -32,7 +32,7 @@
     html.loading.in-dev-mode body::before {
       background-color: #fff;
       border-bottom: 1px solid #e0e0e0;  /* --google-grey-300 */
-      height: 52px;
+      height: 54px;
     }
 
     html[dark].loading.in-dev-mode body::before {

chrome/browser/resources/md_extensions/toolbar.html

@@ -58,7 +58,6 @@
       #devDrawer {
         background: #fff;
         border-bottom: 1px solid var(--google-grey-300);
-        box-sizing: border-box;
         height: 0;
         overflow-x: hidden;
         overflow-y: auto;