This avoids creating a new Data object and copying it around.
Gives a noticable performance increase in the benchmark which
tests validation-only draw calls. Gives about a 8% increase
in the benchmark.

BUG=angleproject:959

Change-Id: Id3a7459753b1b466a06da89f3f8b03b2c2c7a5c1
Reviewed-on: https://chromium-review.googlesource.com/267752Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>