Add NUM_VALUES for enums and size for structs (see #3836)

API versioning requires that enumerations end with a count value
(`*_NUM_VALUES`) and structs begin with a size value (`size_t size`).
Wrapper templates are updated to support structs with different size
values indicating different versions.

To test:
Run `ceftests --gtest_filter=ApiVersionTest.StructVersion*`

libcef/browser/context.cc

@@ -315,7 +315,7 @@ bool CefInitialize(const CefMainArgs& args,
     return true;
   }
 
-  if (settings.size != sizeof(cef_settings_t)) {
+  if (!CEF_MEMBER_EXISTS(&settings, disable_signal_handlers)) {
     DCHECK(false) << "invalid CefSettings structure size";
     return false;
   }