Example build:
https://stainless.corp.google.com/browse/chromeos-autotest-results/277847447-chromeos-test/

Test fails with "KSV failed to draw any windows.". However, the screenshot shows it is painted as expected. It looks like that EnsureClientHasDrawnWindow declares a failure too quickly. We should probably wait for a bit more frames.

Log snippet:
====
2019/01/17 07:43:41 [07:43:41.197] Checking KSV screen
2019/01/17 07:43:41 [07:43:41.197] Waiting for test API extension at chrome-extension://behllobkkfkfnphdnhnkndlbkcpglgmj/_generated_background_page.html
2019/01/17 07:43:42 [07:43:41.923] Connecting to Chrome at ws://127.0.0.1:35452/devtools/page/BE3AC44AB239D89DC868882C751F49BF
2019/01/17 07:43:44 [07:43:44.085] Test API extension is ready
2019/01/17 07:43:44 [07:43:44.296] Error at keyboard_shortcut_viewer_painting.go:75: Failed to check KSV painting: got exception: Error: KSV failed to draw any windows.
2019/01/17 07:43:44 [07:43:44.296] Stack trace:
Failed to check KSV painting
	at chromiumos/tast/local/bundles/cros/ui.KeyboardShortcutViewerPainting (keyboard_shortcut_viewer_painting.go:75)
	at chromiumos/tast/testing.(*Test).Run.func4 (test.go:228)
	at chromiumos/tast/testing.runStages.func1.1 (stage.go:39)
	at chromiumos/tast/testing.runAndRecover.func1 (stage.go:69)
	at runtime.goexit (asm_amd64.s:1333)
got exception: Error: KSV failed to draw any windows.
	at chromiumos/tast/local/chrome.(*Conn).doEval (conn.go:125)
	at chromiumos/tast/local/chrome.(*Conn).EvalPromise (conn.go:104)
	at chromiumos/tast/local/bundles/cros/ui.KeyboardShortcutViewerPainting (keyboard_shortcut_viewer_painting.go:74)
	at chromiumos/tast/testing.(*Test).Run.func4 (test.go:228)
	at chromiumos/tast/testing.runStages.func1.1 (stage.go:39)
	at chromiumos/tast/testing.runAndRecover.func1 (stage.go:69)
	at runtime.goexit (asm_amd64.s:1333)
2019/01/17 07:43:45 Completed test ui.KeyboardShortcutViewerPainting in 15.212s with 1 error(s)