The variable mLocationsUsedForXfbExtension was never initialized
which would give it an undefined value for the lifetime of a
Program. This was causing the capture replay tests to be very
flaky.

Bug: angleproject:5248
Change-Id: Id8d61b88fb2ca2c6340acfe9d6e3720378528987
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2497562Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>