PDF Viewer update: Improve toolbar's vertical alignment.

Fixed: 1123803
Change-Id: Ibf8a56c667ca469d0e4679c7be9e88f682446848
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2389343
Commit-Queue: John Lee <johntlee@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804353}

chrome/browser/resources/pdf/elements/viewer-page-selector.html

     <style>
       :host {
+        align-items: center;
         color: #fff;
         display: flex;
         font-size: 0.81rem;

chrome/browser/resources/pdf/elements/viewer-pdf-toolbar-new.html

@@ -67,6 +67,7 @@
 
   viewer-page-selector {
     display: inline-flex;
+    height: 36px; /* Matching default --cr-icon-button-size */
   }
 
   input {