In deleteTransformFeedback, we were using a const iterator, in a
non-const method. Fix the comparison by using auto, which delegates
more work to the compiler's static type checking.

BUG=angle:773

Change-Id: Ibc362fe49629331811a5302df1b68269dfa128a6
Reviewed-on: https://chromium-review.googlesource.com/238444Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>