Add unittests target to the binary distribution (issue #1632)

- Move all tests from the top-level directory to tests/.
- Move files shared by cefclient and unittests to tests/shared/.
- Add a fused (single header/source file) version of gtest in
  tests/gtest/ with associated CMake configuration.
- Test-only headers are now exposed in include/test/. Unit test
  targets must define UNIT_TEST in order to access them.
- Replace usage of USING_CEF_SHARED with WRAPPING_CEF_SHARED for
  clarity (only the libcef_dll_wrapper target should define it).
- Remove the RENAME_DIRECTORY CMake macro which is no longer used.
- Remove C++11 usage from unittests sources for compatibility with
  the binary distribution configuration.
- Windows: Fix build errors due to chrome_elf.dll and imm32.lib
  missing from the CMake configuration.

libcef_dll/cpptoc/dialog_handler_cpptoc.h

@@ -14,9 +14,9 @@
 #define CEF_LIBCEF_DLL_CPPTOC_DIALOG_HANDLER_CPPTOC_H_
 #pragma once
 
-#ifndef USING_CEF_SHARED
-#pragma message("Warning: "__FILE__" may be accessed wrapper-side only")
-#else  // USING_CEF_SHARED
+#if !defined(WRAPPING_CEF_SHARED)
+#error This file can be included wrapper-side only
+#endif
 
 #include "include/cef_dialog_handler.h"
 #include "include/capi/cef_dialog_handler_capi.h"
@@ -31,5 +31,4 @@ class CefDialogHandlerCppToC
   CefDialogHandlerCppToC();
 };
 
-#endif  // USING_CEF_SHARED
 #endif  // CEF_LIBCEF_DLL_CPPTOC_DIALOG_HANDLER_CPPTOC_H_