This allows us to cache a std::vector between calls, and avoids us
calling allocation/constructors for locals, which saves us some
time. It also allows us to use the vector's size to limit the range
of attribs we look at.

BUG=angleproject:959

Change-Id: I799ed6c92fa8fca96e92e235b125a11d2d551aab
Reviewed-on: https://chromium-review.googlesource.com/277286Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>