This cl makes 2 changes allowing Chromecast tests to use
SwANGLE:
1) It sets angle_use_vulkan_display to true when using
   Chromecast, effectively allowing SwANGLE to be used
2) It sets angle_vulkan_display_mode to "headless", allowing
   it to use DisplayVkHeadless instead of DisplayVkSimple,
   which ends up using "VK_EXT_headless_surface" instead of
   "VK_KHR_display", so that it's supported by SwiftShader
   Vulkan on Chromecast.

Bug: chromium:1060139
Change-Id: Ia521ebddac14f5e34666bb9b64603bec598c41ce
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2826196Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>