cros: Update migration banner on login

For all screens:
- Banner content div fit to text content with max-width 722px.
  This makes the banner fit text better and gets rid of excessive
  left/right padding when text is short;
- Remove font-size override and use inherited 14px from body;

For login screen (where banner is only used for migration message):
- Reduce 4px of banner-pod space (16px between banner bottom and
  the selected user pod with this CL);
- Add 4px left/right padding (24px instead of 20px with the CL);

BUG=715706

Review-Url: https://codereview.chromium.org/2849583002
Cr-Commit-Position: refs/heads/master@{#467758}

ui/login/account_picker/screen_account_picker.css

@@ -18,6 +18,10 @@
   width: 100%;
 }
 
+html[screen=login] #signin-banner-container1 {
+  margin-bottom: 6px;
+}
+
 #signin-banner-container2 {
   display: inline-block;
   position: relative;
@@ -34,12 +38,12 @@ html[dir=rtl] #signin-banner-container2 {
   border-radius: 4px;
   color: whitesmoke;
   display: none;
-  font-size: 15px;
   left: -50%;
+  max-width: 722px;
   padding: 20px;
   position: relative;
   text-align: center;
-  width: 722px;
+  width: max-content;
 }
 
 html[dir=rtl] #signin-banner {
@@ -51,6 +55,10 @@ html[screen=user-adding] #signin-banner {
   display: inline-block;
 }
 
+html[screen=login] #signin-banner {
+  padding: 20px 24px;
+}
+
 html[screen=login] #signin-banner,
 html[screen=lock] #signin-banner {
   display: inline-block;