[Assistant Opt-in] Set line-height for dialog title

Bug: 1124321, b/162780582
Test: Manual Test
Change-Id: I5020cde15baec511782f26df21ea51a3a426ab89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397323
Auto-Submit: Yue Li <updowndota@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804945}

chrome/browser/resources/chromeos/assistant_optin/assistant_shared_styles.css

@@ -44,12 +44,14 @@ webview {
   color: var(--google-grey-900);
   font-family: 'Google Sans', Roboto, sans-serif;
   font-size: 28px;
+  line-height: 32px;
   padding: 20px 0 16px 0;
 }
 
 .sub-title {
   color: var(--google-grey-900);
   font: 13px Roboto, sans-serif;
+  line-height: 16px;
 }
 
 .content {