Themes: Remove unused IDs from ThemeProperties

chrome/test/data/profiles/profile_with_complex_theme/Default/PreferencesTemplate

 {
    "download": {
       "directory_upgrade": true,
       "extensions_to_open": ""
    },
    "extensions": {
       "autoupdate": {
          "next_check": "12894520286433189"
       },
       "settings": {
          "mblmlcbknbnfebdfjnolmcapmdofhmme": {
             "location": 1,
             "manifest": {
                "key": "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDnTeJKFLxjfDfc0gh9WIjyjdGXyKReXsUXxKdtkfsi9mauRlxd9nYwwbMXzJXRkOFyP1zuwhjPkMtHv3oVcITK74YOlFzgQ/RxGkxdaf+Z6Pmh9cu4s4GEtIiY+HmUHYEVBAJEK9xSmSOJI3AxdQnrrjMBd7jCGuJULYlv/Ul7JQIDAQAB",
                "name": "Star Gazing",
                "theme": {
                   "colors": {
                      "bookmark_text": [ 0, 0, 0 ],
                      "frame": [ 57, 137, 194 ],
                      "ntp_background": [ 57, 137, 194 ],
                      "ntp_link": [ 0, 0, 0 ],
                      "ntp_section": [ 131, 138, 146, 0.8 ],
-                     "ntp_section_link": [ 255, 255, 255 ],
-                     "ntp_section_text": [ 255, 255, 255 ],
                      "ntp_text": [ 0, 0, 0 ],
                      "tab_background_text": [ 0, 0, 0 ],
                      "tab_text": [ 0, 0, 0 ],
                      "toolbar": [ 204, 208, 212 ]
                   },
                   "images": {
                      "theme_button_background": "i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUYwsgCDA",
                      "theme_frame": "i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUYw8gCDA",
                      "theme_ntp_background": "i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUYj9gCDA",
                      "theme_tab_background": "i/agxjaHJvbWV0aGVtZXNyDAsSBEZpbGUY_c8CDA",
@@ skipping 26 matching lines @@
       "name": "Not Signed In",
       "nickname": "not-signed-in"
    },
    "session": {
       "startup_urls": [  ]
    },
    "spellcheck": {
       "dictionary": "en-US"
    }
 }