Lock generator_mutex_ inside DecodeFailed

Bug: 962126

We typically lock this mutex before reading or writing to
decode_failed_. Do so in one more place to avoid a data race.

Change-Id: I0248e4dd99defc5ad3d09575096847e6455adfc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610085
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Auto-Submit: Leon Scroggins <scroggo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659475}