Although we've split out a lot of the embedder-specific code between the Win32 and UWP embedder, there are still a lot of #ifdef WINUWP in the code. We should split out embedder-specific code into foo_win32.cc and foo_winuwp.cc for all the remaining cases for a few reasons:

• Consistent handling of embedder-specific code
• Easy to spot which components have embedder-specific implementation by reading the code
• Better developer velocity by reducing the chance of introducing bugs/syntax errors in the embedder you're not working on that are only uncovered once a patch is pushed to CI.