Nux Onboarding: change sign-in page to match mock

- Update string
- fix text not centering on small screens.

Bug: 900378
Change-Id: Iad5640c486974c1e515df9f2519e828098d900da
Reviewed-on: https://chromium-review.googlesource.com/c/1308909
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604780}

chrome/app/onboarding_welcome_strings.grdp

@@ -72,7 +72,7 @@
     Sign in and turn on sync to get your bookmarks, passwords and more on all devices
   </message>
   <message name="IDS_ONBOARDING_WELCOME_SIGNIN_VIEW_SIGNIN" desc="The label for a button to let users sign in to chrome.">
-    Sign in
+    Continue
   </message>
 
   <!-- Email interstitial view -->

chrome/browser/resources/welcome/onboarding_welcome/shared/splash_pages_shared_css.html

@@ -19,6 +19,7 @@
         font-size: 4rem;
         margin-bottom: 40px;
         margin-top: 16px;
+        text-align: center;
       }
 
       h2 {
@@ -28,6 +29,7 @@
         line-height: 2.25rem;
         margin: 0;
         opacity: 0.8;
+        text-align: center;
       }
 
       paper-button {