Setting this environment variable will make ANGLE pop up a dialog. The
dev can then attach a debugger to this dialog. Only implemented on
Windows currently.

This option is only available when ASSERTs are enabled in libGLESv2.

The code is based on SwiftShader's implementation of WaitForDebugger:
https://cs.chromium.org/chromium/src/third_party/swiftshader/src/Vulkan/main.cpp

Bug: angleproject:4072
Change-Id: I160cb91a423a6d4517f067f2a6f3a2d953b26505
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1892173Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>