MD Settings: clicking on the "Learn more" shouldn't select the sync passphrase option

It should only open the learn more link.

R=tommycli@chromium.org
BUG=621702
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2575743002
Cr-Commit-Position: refs/heads/master@{#438710}

chrome/browser/resources/settings/people_page/sync_page.html

@@ -228,7 +228,8 @@
           <div class="list-item">
             <span>
               [[enterPassphrasePrompt_(syncPrefs.passphraseTypeIsCustom)]]
-              <a href="$i18nRaw{syncErrorHelpUrl}" target="_blank">
+              <a href="$i18nRaw{syncErrorHelpUrl}" target="_blank"
+                  on-tap="onSyncLearnMoreTap_">
                 $i18n{learnMore}
               </a>
             </span>

chrome/browser/resources/settings/people_page/sync_page.js

@@ -291,6 +291,14 @@ Polymer({
     return passphrase !== '' && confirmation !== '';
   },
 
+  /**
+   * @param {!Event} e
+   * @private
+   */
+  onSyncLearnMoreTap_: function(e) {
+    e.stopPropagation();
+  },
+
   /**
    * Sends the newly created custom sync passphrase to the browser.
    * @private