Add custom search engines list in Search Engine Settings.

This CL changes how search engines are displayed in Search Engine Settings.

Previous(1 list):
  1. search engines that are prepopulated or created by policy.

Current(2 lists):
  1. Search engines that are prepopulated or created by policy, and possibly
     one custom search engine at the end if it's selected as default search
     engine;
  2. All remaining search engines.

Unittests are also updated.

Bug: 433824
Change-Id: Ib70a18f0544101760c331bb349ee9818956ed231
Reviewed-on: https://chromium-review.googlesource.com/c/1323509Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606776}

ios/chrome/app/strings/ios_strings.grd

@@ -1313,6 +1313,9 @@ Handoff must also be enabled in the General section of Settings, and your device
       <message name="IDS_IOS_SEARCH_ENGINE_SETTING_TITLE" desc="The title for Search Engine selection setting [iOS only]">
         Search Engine
       </message>
+      <message name="IDS_IOS_SEARCH_ENGINE_SETTING_CUSTOM_SECTION_HEADER" desc="The header for custom search engines section in Search Engine selection setting [iOS only]">
+        Recently Visited
+      </message>
       <message name="IDS_IOS_SETTINGS_SITE_COPY_BUTTON" desc="Button that the user can press to copy the site to clipboard. [Length: 12em]">
         Copy
       </message>