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/shared/common/client_switches.cc

@@ -38,6 +38,7 @@ const char kHideTopMenu[] = "hide-top-menu";
 const char kWidevineCdmPath[] = "widevine-cdm-path";
 const char kSslClientCertificate[] = "ssl-client-certificate";
 const char kCRLSetsPath[] = "crl-sets-path";
+const char kLoadExtension[] = "load-extension";
 
 }  // namespace switches
 }  // namespace client

tests/shared/common/client_switches.h

@@ -32,6 +32,7 @@ extern const char kHideTopMenu[];
 extern const char kWidevineCdmPath[];
 extern const char kSslClientCertificate[];
 extern const char kCRLSetsPath[];
+extern const char kLoadExtension[];
 
 }  // namespace switches
 }  // namespace client