Allow Chromecast tests to run on SwANGLE 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/+/2826196 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Alexis Hétu <sugoi@chromium.org>