Until late 2017, ANGLE supported  Windows Store apps on Windows 8.1,
Windows Phone 8.1, and Windows 10 (via the Universal Windows
Platform, aka UWP).

Unfortunately ANGLE deprecated support for Windows Store when it
switched from GYP to GN in 2017. Since then, users have been able
to use Microsoft\angle for their UWP apps but this isn't ideal since
it's based on a 2017 copy of Google\angle.

This PR bring back support for UWPs, so that UWP users can use
Google\angle again. Specifically it:

- Adds support for generating UWP projects via GN
- Adds helper/util functions specific to UWP (they're mostly
similar to the desktop Windows helpers)
- Fixes some existing Windows Store code that's rotted since 2017
- Disables async shader compilation for UWPs, since its
implementation calls wait on the UI thread (which is forbidden
in UWPs)
- Renames 'ANGLE_ENABLE_WINDOWS_STORE' to
'ANGLE_ENABLE_WINDOWS_UWP', since ANGLE only support UWPs now
- Fixes misc other related issues (such as dependencies on D3D9
headers in API-agnostic code)

Note that this doesn't bring back support for Windows/Phone 8.1.

BUG=angleproject:3922

Change-Id: Ia79ae05a5e0e0a0625eb633bf1928722dfd3e85f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1811871
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>