AW UMA: remove PrefLoadTime histogram

No change to logic. This removes the Android.WebView.PrefLoadTime
histogram. Based on the metrics we've received so far, pref loading time
does not seem especially sensitive to the number of prefs in the
persistence allowlist. Since we're no longer concerned about new prefs
impacting startup, this removes the histogram.

This histogram was originally added in https://crrev.com/c/1956079.

This also removes the "histogram_macros.h" import since the
corresponding logging code was removed a while ago in
https://crrev.com/c/2247154.

Fixed: 1140013
Test: N/A
Change-Id: I2ba6d05f23e4be36914da733d7810adfeab9cfca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512386Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823767}

android_webview/browser/aw_feature_list_creator.cc

@@ -21,8 +21,6 @@
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
-#include "base/metrics/histogram_functions.h"
-#include "base/metrics/histogram_macros.h"
 #include "base/path_service.h"
 #include "base/strings/string_split.h"
 #include "base/time/time.h"
@@ -144,13 +142,7 @@ std::unique_ptr<PrefService> AwFeatureListCreator::CreatePrefService() {
   pref_service_factory.set_read_error_callback(
       base::BindRepeating(&HandleReadError));
 
-  base::TimeTicks pref_load_start = base::TimeTicks::Now();
-  auto service = pref_service_factory.Create(pref_registry);
-  base::TimeDelta pref_load_time = base::TimeTicks::Now() - pref_load_start;
-  UmaHistogramCustomTimes("Android.WebView.PrefLoadTime", pref_load_time,
-                          base::TimeDelta::FromMilliseconds(1),
-                          base::TimeDelta::FromMinutes(1), 50);
-  return service;
+  return pref_service_factory.Create(pref_registry);
 }
 
 void AwFeatureListCreator::SetUpFieldTrials() {

tools/metrics/histograms/histograms_xml/android/histograms.xml

@@ -2557,6 +2557,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Android.WebView.PrefLoadTime" units="ms"
     expires_after="2020-12-01">
+  <obsolete>
+    Removed in November 2020.
+  </obsolete>
   <owner>rmcelrath@chromium.org</owner>
   <owner>src/android_webview/OWNERS</owner>
   <summary>