Add support for loading extensions (issue #1947)

- Add CefRequestContext::LoadExtension, CefExtension, CefExtensionHandler and
  related methods/interfaces.
- Add chrome://extensions-support that lists supported Chrome APIs.
- Add CefBrowserHost::SetAutoResizeEnabled and CefDisplayHandler::OnAutoResize
  to support browser resize based on preferred web contents size.
- views: Add support for custom CefMenuButton popups.
- cefclient: Run with `--load-extension=set_page_color` command-line flag for
  an extension loading example. Add `--use-views` on Windows and Linux for an
  even better example.

tests/ceftests/scheme_handler_unittest.cc

@@ -161,7 +161,8 @@ class TestSchemeHandler : public TestHandler {
 #if defined(OS_LINUX)
     // CustomStandardXHR* tests are flaky on Linux, sometimes returning
     // ERR_ABORTED. Make the tests less flaky by also accepting that value.
-    if (!(test_results_->expected_error_code == 0 && errorCode == ERR_ABORTED)) {
+    if (!(test_results_->expected_error_code == 0 &&
+          errorCode == ERR_ABORTED)) {
       EXPECT_EQ(test_results_->expected_error_code, errorCode);
     }
 #else