Fix -Wshadow-field warning in class NullInterface

This warning fires in a tip of tree clang build because the base class to NullInterface also has a field called fExtensions.

Change-Id: I5e2a901583ae223a85d562f2d8b082e4d0cbf690
Reviewed-on: https://skia-review.googlesource.com/8691
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed