Set the flag quic_enable_ack_decimation to true.

While ACK decimation is not currently enabled by default,
it *is* enabled via experiments ubiquitously. This may cause
a regression in some perf tests, but those tests are not
running with the config used in production (with ACK
decimation enabled).

b/79972781

BUG=842677

Change-Id: Ie2729bd8c47f9d9781e5de6aa39379c740319976
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015495
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Auto-Submit: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734231}

net/quic/quic_flags_list.h

@@ -85,7 +85,7 @@ QUIC_FLAG(double, FLAGS_quic_lumpy_pacing_cwnd_fraction, 0.25f)
 
 // Default enables QUIC ack decimation and adds a connection option to disable
 // it.
-QUIC_FLAG(bool, FLAGS_quic_reloadable_flag_quic_enable_ack_decimation, false)
+QUIC_FLAG(bool, FLAGS_quic_reloadable_flag_quic_enable_ack_decimation, true)
 
 // If true, QUIC offload pacing when using USPS as egress method.
 QUIC_FLAG(bool, FLAGS_quic_restart_flag_quic_offload_pacing_to_usps2, false)