cefclient: win: Add code signing verification (see #3935)

Move code signing verification code to libcef_dll_wrapper
and add example checks in cefclient.

Load libcef.dll with code signing checks.

Add a CefScopedLibraryLoader variant for Windows.

libcef_dll/bootstrap/bootstrap_util_win.h

@@ -19,8 +19,9 @@ class CommandLine;
 namespace bootstrap_util {
 
 namespace switches {
+// Changes to this value require rebuilding libcef.dll.
 inline constexpr char kModule[] = "module";
-}
+}  // namespace switches
 
 // Returns true if |name| is one of the default bootstrap executable names.
 bool IsDefaultExeName(const std::wstring& name);
@@ -47,8 +48,6 @@ std::wstring GetDefaultModuleValue(const base::FilePath& exe_path);
 bool IsModulePathAllowed(const base::FilePath& module_path,
                          const base::FilePath& exe_path);
 
-std::wstring GetLastErrorAsString();
-
 }  // namespace bootstrap_util
 
 #endif  // CEF_LIBCEF_DLL_BOOTSTRAP_BOOTSTRAP_UTIL_WIN_H_