[threadpool] Report data from ThreadPool.NumTasksRunWhileQueuing.*

This adds a historical_histogram_data.md file in which data from the
ThreadPool.NumTasksRunWhileQueuing.* histogram is reported. Actions to
improve the situation described by this histogram are recommended.

Bug: 906079
Change-Id: I4872b6e0c38979161547c8e40ff3125bb971d50a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804789
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698464}

base/task/thread_pool/historical_histogram_data.md

+# Historical Histogram Data
+
+This page presents data captured from `base::ThreadPool` histograms at a given
+point in time so it can be used in future design decisions.
+
+All data is 28-day aggregation on Stable channel.
+
+## Number of tasks between waits
+
+Number of tasks between two waits by a foreground worker thread in a
+browser/renderer process.
+
+Histogram name: ThreadPool.NumTasksBetweenWaits.(Browser/Renderer).Foreground
+Date: August 2019
+Values in tables below are percentiles.
+
+### Windows
+
+| Number of tasks | Browser process | Renderer process |
+|-----------------|-----------------|------------------|
+| 1               | 87              | 92               |
+| 2               | 95              | 98               |
+| 5               | 99              | 100              |
+
+### Mac
+
+| Number of tasks | Browser process | Renderer process |
+|-----------------|-----------------|------------------|
+| 1               | 81              | 90               |
+| 2               | 92              | 97               |
+| 5               | 98              | 100              |
+
+### Android
+
+| Number of tasks | Browser process | Renderer process |
+|-----------------|-----------------|------------------|
+| 1               | 92              | 96               |
+| 2               | 97              | 98               |
+| 5               | 99              | 100              |
+
+
+## Number of tasks run while queueing
+
+Number of tasks run by ThreadPool while task was queuing (from time task was
+posted until time it was run). Recorded for dummy heartbeat tasks in the
+*browser* process. The heartbeat recording avoids dependencies between this
+report and other work in the system.
+
+Histogram name: ThreadPool.NumTasksRunWhileQueuing.Browser.*
+Date: September 2019
+Values in tables below are percentiles.
+
+Note: In *renderer* processes, on all platforms/priorities, 0 tasks are run
+while queuing at 99.5th percentile.
+
+### Windows
+
+| Number of tasks | USER_BLOCKING | USER_VISIBLE | BEST_EFFORT |
+|-----------------|---------------|--------------|-------------|
+| 0               | 95            | 93           | 90          |
+| 1               | 98            | 95           | 92          |
+| 2               | 99            | 96           | 93          |
+| 5               | 100           | 98           | 95          |
+
+### Mac
+
+| Number of tasks | USER_BLOCKING | USER_VISIBLE | BEST_EFFORT |
+|-----------------|---------------|--------------|-------------|
+| 0               | 100           | 100          | 99          |
+| 1               | 100           | 100          | 99          |
+| 2               | 100           | 100          | 99          |
+| 5               | 100           | 100          | 100         |
+
+### Android
+
+| Number of tasks | USER_BLOCKING | USER_VISIBLE | BEST_EFFORT |
+|-----------------|---------------|--------------|-------------|
+| 0               | 99            | 98           | 97          |
+| 1               | 100           | 99           | 99          |
+| 2               | 100           | 99           | 99          |
+| 5               | 100           | 100          | 100         |
+
+### Chrome OS
+
+For all priorities, 0 tasks are run while queueing at 99.5th percentile.
+
+### Analysis
+
+The number of tasks that run while a BEST_EFFORT task is queued is unexpectedly
+low. We should explore creating threads less aggressively, at the expense of
+keeping BEST_EFFORT tasks in the queue for a longer time. See
+[Bug 906079](https://crbug.com/906079).

base/task/thread_pool/thread_group_impl.cc

@@ -41,25 +41,6 @@
 #include "base/win/windows_version.h"
 #endif  // defined(OS_WIN)
 
-// Data from deprecated UMA histograms:
-//
-// ThreadPool.NumTasksBetweenWaits.(Browser/Renderer).Foreground, August 2019
-//   Number of tasks between two waits by a foreground worker thread in a
-//   browser/renderer process.
-//
-//  Windows (browser/renderer)
-//    1 at 87th percentile / 92th percentile
-//    2 at 95th percentile / 98th percentile
-//    5 at 99th percentile / 100th percentile
-//  Mac (browser/renderer)
-//    1 at 81th percentile / 90th percentile
-//    2 at 92th percentile / 97th percentile
-//    5 at 98th percentile / 100th percentile
-//  Android (browser/renderer)
-//    1 at 92th percentile / 96th percentile
-//    2 at 97th percentile / 98th percentile
-//    5 at 99th percentile / 100th percentile
-
 namespace base {
 namespace internal {