VertexAttributeTest: don't copy too much data in the buffer
VertexAttributeTest: don't copy too much data in the buffer

BUG=angleproject:1570

Change-Id: I199bb88042c3926cb05e27494bb6af9b2666cd00
Reviewed-on: https://chromium-review.googlesource.com/402932
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

ppy/angle,MSOpenTech/angle,MSOpenTech/angle,ecoal95/angle,ecoal95/angle,MSOpenTech/angle,ecoal95/angle,ppy/angle,ecoal95/angle,ecoal95/angle,MSOpenTech/angle,ppy/angle,ppy/angle