vp8_impl.cc: Rename postproc field trial string for arm.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/3006143002
Cr-Original-Commit-Position: refs/heads/master@{#19713}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a505fb021f62898d282b8eae072dd73ee1eccc6a