Avoid possible overflow when computing ReadPixels bounds. BUG=809881 Change-Id: I69c58c93597aea5a5c25298908592ce1ab4a957b Reviewed-on: https://chromium-review.googlesource.com/929984 Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>