The overhead of the function call ended up being a hot spot for draw calls
since the attributes are iterated over many times in VertexDataManager.

BUG=angleproject:959

Change-Id: I9bbfcbd115661ad629db9ed93d683cd8d0dc9a78
Reviewed-on: https://chromium-review.googlesource.com/263102Tested-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>