Vulkan: Set content undefined on eglSwapBuffers * Invalidate the swap buffer content if the swap behavior is EGL_BUFFER_DESTROYED. * In that event, WindowSurfaceVk::getBufferAge() sets `age` to 0 and returns with no errors. * Unit tests in EGLBufferAgeTest.cpp updated to reflect this. * Minor cleanup in the test file. Bug: angleproject:6363 Change-Id: Ibab7ccb99c1be149588e6aad16bcc2d6dd989295 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3307776 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Amirali Abdolrashidi <abdolrashidi@google.com>