Use auto, not const auto*, for libyuv function pointers

This was a compiler error on GCC 4.9.1:
unable to deduce ‘const auto*’ from ‘((((content::PpFrameWriter::FrameWriterDelegate*)this)->content::PpFrameWriter::FrameWriterDelegate::endian_ == (content::PpFrameWriter::FrameWriterDelegate::PixelEndian)1u) ? libyuv::BGRAToI420 :  libyuv::ARGBToI420)’

Introduced in https://codereview.chromium.org/663183002

BUG=426020

Review URL: https://codereview.chromium.org/675963002

Cr-Commit-Position: refs/heads/master@{#301083}
1 file changed