Navi: Update centering CSS to prevent clipping on small screen.

Screenshot in bug.

Bug: 928174
Change-Id: I55fb4d0226fd623a4d377ed95928891744e55cef
Reviewed-on: https://chromium-review.googlesource.com/c/1459578Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630114}

chrome/browser/resources/welcome/onboarding_welcome/welcome_app.html

@@ -19,11 +19,8 @@
   <template>
     <style include="paper-button-style cr-hidden-style">
       #viewManager {
-        align-items: center;
         display: flex;
-        flex-direction: column;
         font-size: 100%;
-        justify-content: center;
         margin: 0;
         min-height: 100vh;
       }
@@ -33,6 +30,8 @@
         /* Override cr-view-manager's default styling for view. */
         bottom: initial;
         left: initial;
+        margin: auto;
+        position: unset;
         right: initial;
         top: initial;
       }