Switch from WebDocument::insertStyleText to ::insertUserStyleSheet for programatic CSS injection.

chrome/test/data/extensions/api_test/executescript/basic/test.html

 <script>
 
 var pass = chrome.test.callbackPass;
 var fail = chrome.test.callbackFail;
 var assertEq = chrome.test.assertEq;
 var assertTrue = chrome.test.assertTrue;
 var relativePath =
     '/files/extensions/api_test/executescript/basic/test_executescript.html';
 var testUrl = 'http://a.com:PORT' + relativePath;
 var testFailureUrl = 'http://b.com:PORT' + relativePath;
@@ skipping 54 matching lines @@
           var script_file = {};
           script_file.file = 'script2.js';
           chrome.tabs.executeScript(tabId, script_file, function() {
             chrome.tabs.get(tabId, pass(function(tab) {
               assertEq(tab.title, 'block');
             }));
           });
         });
       },
 
+      function insertCSSTextShouldNotAffectDOM() {
+        chrome.tabs.insertCSS(tabId, {code: 'p {display: none}'}, function() {
+          chrome.tabs.executeScript(
+              tabId,
+              {code: 'document.title = document.styleSheets.length'},
+              function() {
+                chrome.tabs.get(tabId, pass(function(tab) {
+                  assertEq(tab.title, '0');
+                }));
+             });
+        });
+      },
+
       function executeJavaScriptCodeShouldFail() {
         chrome.tabs.update(tabId, { url: testFailureUrl }, function() {
           var script_file = {};
           script_file.code = "document.title = 'executeScript';";
           chrome.tabs.executeScript(tabId, script_file, fail(
               'Cannot access contents of url "' + testFailureUrl +
               '". Extension manifest must request permission to access this ' +
               'host.'));
         });
       },
@@ skipping 12 matching lines @@
             'Code and file should not be specified ' +
             'at the same time in the second argument.'));
       }
     ]);
   });
 
   chrome.tabs.create({ url: testUrl });
 });
 
 </script>