Like bug 821521, it looks like this is probably related to the loading metric changes introduced in https://chromium-review.googlesource.com/939481 (and now being reverted). 

The stack trace I'm seeing is:

(WARNING) 2018-03-12 18:56:48,049 timeline_based_measurement._ComputeTimelineBasedMetrics:316  Processing resulting traces took 4.146 seconds
Failure recorded: Error: Breakdown maps from strings to numbers
    at Breakdown.set (/tracing/value/diagnostics/breakdown.html:49:15)
    at createBreakdownDiagnostic (/tracing/metrics/system_health/loading_metric.html:63:27)
    at collectTimeToEvent (/tracing/metrics/system_health/loading_metric.html:157:22)
    at collectLoadingMetricsForRenderer (/tracing/metrics/system_health/loading_metric.html:277:31)
    at new loadingMetric (/tracing/metrics/system_health/loading_metric.html:429:11)
    at runMetrics (c:\b\s\w\ir\third_party\catapult\tracing\tracing\metrics\metric_map_function.html:47:16)
    at metricMapFunction (c:\b\s\w\ir\third_party\catapult\tracing\tracing\metrics\metric_map_function.html:150:24)
    at Object.mapSingleTrace (/tracing/mre/map_single_trace.html:39:7)
    at eval (c:\b\s\w\ir\third_party\catapult\tracing\tracing\mre\map_single_trace_cmdline.html:61:18)
    at Object.runAndConvertErrorsToFailures (/tracing/mre/map_single_trace.html:24:10)

The corresponding trace is at https://console.developers.google.com/m/cloudstorage/b/chrome-telemetry-output/o/yahoo_co_jp_2018-03-12_18-56-26_29486.html. To reproduce it, open up the metrics side panel and click "loadingMetric", which should produce an error.

It looks like this is happening inconsistently on various stories.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6aea1cac122502499713bfc182d0454d0f325fa9

commit 6aea1cac122502499713bfc182d0454d0f325fa9
Author: catapult-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com <catapult-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Date: Fri Mar 23 14:48:06 2018

Roll src/third_party/catapult/ 734f737c6..c4e9b1332 (14 commits)

https://chromium.googlesource.com/catapult.git/+log/734f737c6b57..c4e9b1332818

$ git log 734f737c6..c4e9b1332 --date=short --no-merges --format='%ad %ae %s'
2018-03-23 nednguyen Revert "Add --enable-automation flag in GetFromBrowserOptions()"
2018-03-22 dtu [pinpoint] Set auto_explore == True for "patch jobs".
2018-03-21 ynovikov Remove a note on android_optional_gpu_tests_rel from manual rolls doc.
2018-03-15 dproy Reland "More precise self time calculation"
2018-03-21 nednguyen Ignore fetchts file
2018-03-20 laszio [Telemetry] Support outputing json in benchmark_runner
2018-03-19 kjharland [Dashboard] Whitelist prod Fuchsia Garnet builder.
2018-03-20 benjhayden Fix sorting results.html
2018-03-19 horo Add --enable-automation flag in GetFromBrowserOptions()
2018-03-19 eakuefner [Dashboard] Avoid creating empty rows/histograms/tests if histograms are empty
2018-03-16 dtu [pinpoint] Infer "repository" parameter from configuration.
2018-03-19 chiniforooshan First set of thread times metrics in TBMv2
2018-03-16 erikchen Replace --enable-heap-profiling with --memlog equivalent.
2018-03-19 wangxianzhu Fix visual rect support for cc::DisplayItemList

Created with:
  roll-dep src/third_party/catapult
BUG=chromium:822258, chromium:786572 , chromium:731979 ,chromium:821531,chromium:821521, chromium:819969 ,chromium:822258, chromium:627461 , chromium:822843 


The AutoRoll server is located here: https://catapult-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


TBR=sullivan@chromium.org

Change-Id: I8c0025df6198bf9b1662bb8d78e45741d66f6aff
Reviewed-on: https://chromium-review.googlesource.com/977884
Commit-Queue: catapult-chromium-autoroll <catapult-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Reviewed-by: catapult-chromium-autoroll <catapult-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#545441}
[modify] https://crrev.com/6aea1cac122502499713bfc182d0454d0f325fa9/DEPS