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/tracing_unittest.cc

@@ -8,9 +8,9 @@
 #include "include/cef_trace.h"
 #include "include/cef_waitable_event.h"
 #include "include/wrapper/cef_closure_task.h"
-#include "tests/ceftests/file_util.h"
 #include "tests/ceftests/test_handler.h"
 #include "tests/gtest/include/gtest/gtest.h"
+#include "tests/shared/browser/file_util.h"
 
 // Use the CEF version of the TRACE_* macros instead of the Chromium version.
 #undef USING_CHROMIUM_INCLUDES
@@ -81,7 +81,7 @@ class TracingTestHandler : public CefEndTracingCallback,
   void ReadTracingFile(const std::string& file_path) {
     EXPECT_FILE_THREAD();
 
-    EXPECT_TRUE(file_util::ReadFileToString(file_path, &trace_data_));
+    EXPECT_TRUE(client::file_util::ReadFileToString(file_path, &trace_data_));
     EXPECT_TRUE(CefDeleteFile(file_path, false));
 
     completion_event_->Signal();