We were passing an unsigned int mask instead of a GLenum drawMode. This
was caught by converting draw modes to internal packed enums.

Bug: angleproject:2574
Change-Id: I4b063590aa60381b5ffa5eba0479a86d10b439c6
Reviewed-on: https://chromium-review.googlesource.com/1067112
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>