WebGLCompatibility: Add test for "negative" offset in DrawElements BUG=angleproject:1523 BUG=chromium:668223 Change-Id: I2d21c15b53fa204b3cb2b0be849cfe91ca63046b Reviewed-on: https://chromium-review.googlesource.com/435884 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>