Reduce the sampling rate for Blink_UpdateTime UKM

The metric is sending twice as much data as it should, and we
are about to add a corresponding compositor latency metric. Hence
we need to reduce the sampling rate by a factor of 4.

Bug: 1035531
Change-Id: If3dfcce9db624d28495c1f44c4887f55cf004326
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974610
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726161}

third_party/blink/renderer/core/frame/local_frame_ukm_aggregator.h

@@ -319,7 +319,7 @@ class CORE_EXPORT LocalFrameUkmAggregator
   // multiplier value should be tuned to achieve a total sample count that
   // avoids throttling by the UKM system.
   double sample_decay_rate_ = 3;
-  double sample_rate_multiplier_ = 1;
+  double sample_rate_multiplier_ = 4;
   unsigned samples_so_far_ = 0;
   unsigned frames_to_next_event_ = 0;