Windows: Add ARM64 distribution support (see issue #2773)

To generate Windows ARM64 builds set the CEF_ENABLE_ARM64=1 environment
variable and pass the "--arm64-build --build-target=cefsimple" command-line
arguments to automate-git.py. We can't build cefclient.exe for ARM64 due to
missing OpenGL library support so build cefsimple.exe instead.

tools/cefbuilds/cef_json_builder.py

@@ -100,7 +100,7 @@ class cef_json_builder:
   def get_platforms():
     """ Returns the list of supported platforms. """
     return ('linux32', 'linux64', 'linuxarm', 'linuxarm64', 'macosarm64', 'macosx64',
-            'windows32', 'windows64')
+            'windows32', 'windows64', 'windowsarm64')
 
   @staticmethod
   def get_distrib_types():