Consolidate a lot of the code duplication in GetIntegerv, GetFloatv,
GetBooleanv and GetInteger64v.

BUG=angle:540

Change-Id: I7b4a5803ea76e5eeb0ebf25c5caadec4c647b485
Reviewed-on: https://chromium-review.googlesource.com/180971
Commit-Queue: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>