Implement experimental.contentSettings.misc.blockThirdPartyCookies.

chrome/test/data/extensions/api_test/content_settings/incognito/test.html

+<script>
+// Content settings API test
+// Run with browser_tests --gtest_filter=ExtensionApiTest.ContentSettings
+
+var cs = chrome.experimental.contentSettings;
+function expect(expected, message) {
+  return chrome.test.callbackPass(function(value) {
+    chrome.test.assertBool(value, expected, message);
+  });
+}
+chrome.test.runTests([
+  function testIncognito() {
+    cs.misc.blockThirdPartyCookies.get(
+        {}, expect(false, "third-party cookies should not be blocked"));
+    cs.misc.blockThirdPartyCookies.get(
+        { 'incognito': true },
+        expect(false,
+               "third-party cookies should not be blocked in incognito mode"));
+    cs.misc.blockThirdPartyCookies.set(
+        { 'incognito': true, 'value': true }, chrome.test.callbackPass());
+    cs.misc.blockThirdPartyCookies.get(
+        {}, expect(false, "third-party cookies should not be blocked"));
+    cs.misc.blockThirdPartyCookies.get(
+        { 'incognito': true },
+        expect(true,
+               "third-party cookies should be blocked in incognito mode"));
+  },
+]);
+</script>