Improves draw call overhead of RendererGL.

DrawCallPerf_gl:
Before: 129779 score
After: 136973 score
Improvement: 5.543%

No noticeable difference in DLL size or draw call perf of the D3D
renderers.

BUG=angleproject:959

Change-Id: Id54d49e9e2cfb69431ee26d632c58fee2c42b82c
Reviewed-on: https://chromium-review.googlesource.com/275408Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>