The "default" platform type isn't really a discrete type, and treating it
that way leads to duplication. Make the default platform just map to
another platform enum.

After this change, setting args to angle_enable_opengl=false
angle_enable_vulkan=true under X11 results in a default of vulkan;
previously we would hit UNREACHABLE() due to not handling this case.

Bug: angleproject:3847
Change-Id: I1dc093ef8a2f5b93721cb1ebda0a8c3ec4a7d2f9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1521321Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>