Changed from 10 to 68.
This is to avoid a flake where the limit is exceeded, see
crbug.com/638554. Our performance tests should flag performance
regressions, we shouldn't rely on crashing because the number of
referenced buffers is not tiny to detect this. However, if a really big
number of buffers (>68) are referenced without being dereferenced it is
likely that we have a bug and frames are leaking in which case we can
DCHECK-crash.
BUG=chromium:638554
Review-Url: https://codereview.webrtc.org/2280593002
Cr-Commit-Position: refs/heads/master@{#14084}