The implementation of DrawElementsIndirect is similar with
DrawArraysIndirect except that it needs to apply IndexBuffer.

BUG=angleproject:1595
TEST=dEQP-GLES31.functional.draw_indirect.draw_elements_indirect*
     dEQP-GLES31.functional.draw_indirect.instancing.*
     dEQP-GLES31.functional.draw_indirect.random.*

Change-Id: I5d2c8a7485b18b724fdda6fd964013c941e45b4f
Reviewed-on: https://chromium-review.googlesource.com/455520
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>