It seems that the compiler didn't resolve the overloaded operator<< with
the structure's grandparent structure. Fix it by explicitely casting to
the grandparent before calling operator<<. Also provides slightly more
information on the number of iterations/triangles.

This wasn't caught with local testing as IndexConversion isn't compiled in
angle_perftests on Linux.

BUG=angleproject:1153
BUG=530226

Change-Id: Ifa602eb0728d052bc651f0dd030f9f880c00dc51
Reviewed-on: https://chromium-review.googlesource.com/298860Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>