Scale down a slow index buffer perf test.
Scale down a slow index buffer perf test.

This test was slow enough that running 20k iterations might have been
causing a timeout on the Nexus 5.

Bug: angleproject:2923 
Change-Id: I03f6e95135d8c521e6ef3e81bc9a4b0dfd7f19df
Reviewed-on: https://chromium-review.googlesource.com/c/1310693
Reviewed-by: Yuly Novikov <36f5580f63d0a3eacc17e89a61d11e2f646630d2@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle