Fix iterator comparison in Context.cpp.
Fix iterator comparison in Context.cpp.

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/238444
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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