OSWindow: add optional debugging of events received BUG=angleproject:830 Change-Id: Icd854ee152f15ec1de4293b5ed2ed49cb1a5fc2b Reviewed-on: https://chromium-review.googlesource.com/270451 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>