Improves draw call overhead of RendererGL.

DrawCallPerf_gl:
Before: 136973 score
After: 153317 score
Improvement: 11.932%

BUG=angleproject:959

Change-Id: Ib75f6fdd756648e4a07f6e970cda03abbdbcf009
Reviewed-on: https://chromium-review.googlesource.com/275409Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>