Implement EGL_KHR_surfaceless_context for D3D, GL and NULL backends. Skip all config attributes that have the value of EGL_DONT_CARE. From the EGL 1.5 spec: "If EGL_DONT_CARE is specified as an attribute value, then the attribute will not be checked.". BUG=angleproject:1651 Change-Id: I30c95a1970543fb6f1d4b02d2babf3df61cad543 Reviewed-on: https://chromium-review.googlesource.com/533937 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>