Document that the DRP pref migration code shouldn't be removed.

This CL removes a TODO suggesting that the migration code should be
removed, replacing it with a comment describing why the migration code
should not be removed.

Bug: 445599
Change-Id: I78660240d3ed957bd76e3f3dbc2475dccba1751c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1737327
Commit-Queue: Scott Little <sclittle@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Auto-Submit: Scott Little <sclittle@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684098}

chrome/browser/data_reduction_proxy/data_reduction_proxy_chrome_settings.cc

@@ -246,7 +246,12 @@ void DataReductionProxyChromeSettings::InitDataReductionProxySettings(
   data_reduction_proxy::DataReductionProxySettings::
       SetCallbackToRegisterSyntheticFieldTrial(base::Bind(
           &ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial));
-  // TODO(bengr): Remove after M46. See http://crbug.com/445599.
+  // In M35 and earlier, the Data Reduction Proxy enabled/disabled setting was
+  // stored in prefs, so this setting needs to be migrated to the new way of
+  // storing the setting. Removing this migration code would cause users
+  // upgrading from M35 and earlier with the Data Reduction Proxy enabled to be
+  // unable to browse non-SSL sites for the most part (see
+  // http://crbug.com/476610).
   MigrateDataReductionProxyOffProxyPrefs(profile_prefs);
 }