generate_vulkan_layers_json.py: Fix Python 3 compatibility

generate_vulkan_layers_json.py: Fix Python 3 compatibility

json.dump() returns a string. Writing strings to a file opened in binary mode doesn't work. Python 2 doesn't properly distringuish bytes and strings.
Open in non-binary mode, so this code works on both.

Traceback (most recent call last):
  File "../../third_party/angle/third_party/vulkan-tools/src/build-gn/generate_vulkan_layers_json.py", line 126, in <module>
    sys.exit(main())
  File "../../third_party/angle/third_party/vulkan-tools/src/build-gn/generate_vulkan_layers_json.py", line 83, in main
    json.dump(data, outfile)
  File "C:\Program Files\Python38\lib\json\__init__.py", line 180, in dump
    fp.write(chunk)
TypeError: a bytes-like object is required, not 'str'

Chromium bug: https://crbug.com/941669