Enable CSP by default for extensions with manifest_version >= 2.

chrome/test/data/extensions/api_test/default_content_security_policy/test.js

+window.externalCanary = "Alive";
+
+chrome.test.getConfig(function(config) {
+
+  function inlineScriptDoesNotRun() {
+    chrome.test.assertEq(window.inlineCanary, undefined);
+    chrome.test.runNextTest();
+  }
+
+  function externalScriptDoesRun() {
+    // This test is somewhat zen in the sense that if external scripts are
+    // blocked, we don't be able to even execute the test harness...
+    chrome.test.assertEq(window.externalCanary, "Alive");
+    chrome.test.runNextTest();
+  }
+
+  chrome.test.runTests([
+    inlineScriptDoesNotRun,
+    externalScriptDoesRun
+  ]);
+});