Always add the ChromeCleanupHandler to the settings WebUI.

Due to loadTimeData inconsistencies between refreshes, it's possible for
"chromeCleanupEnabled" to become true while no ChromeCleanupHandler is
present. 
To avoid showing a blank card or crashing in debug mode for edge cases,
always install the handler.

Also added some documentation to load_time_data.js to hopefully help
other developers avoid this issue.

Bug: 690020
Change-Id: Ifa8e0c0e78c2d6202e11fd378c74781b24401918
Reviewed-on: https://chromium-review.googlesource.com/565327Reviewed-by: Tommy Li <tommycli@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: proberge <proberge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485391}

chrome/browser/ui/webui/settings/md_settings_ui.cc

@@ -180,10 +180,11 @@ MdSettingsUI::MdSettingsUI(content::WebUI* web_ui, const GURL& url)
   html_source->AddString("hostname", url.host());
 
 #if defined(OS_WIN)
-  if (base::FeatureList::IsEnabled(safe_browsing::kInBrowserCleanerUIFeature) &&
-      safe_browsing::ChromeCleanerController::ShouldShowCleanupInSettingsUI()) {
+  if (base::FeatureList::IsEnabled(safe_browsing::kInBrowserCleanerUIFeature)) {
     AddSettingsPageUIHandler(base::MakeUnique<ChromeCleanupHandler>(profile));
-    html_source->AddBoolean("chromeCleanupEnabled", true);
+
+    if (safe_browsing::ChromeCleanerController::ShouldShowCleanupInSettingsUI())
+      html_source->AddBoolean("chromeCleanupEnabled", true);
   }
 #endif  // defined(OS_WIN)
 

ui/webui/resources/js/load_time_data.js

@@ -8,6 +8,10 @@
  * content has finished loading). This data includes both localized strings and
  * any data that is important to have ready from a very early stage (e.g. things
  * that must be displayed right away).
+ *
+ * Note that loadTimeData is not guaranteed to be consistent between page
+ * refreshes (https://crbug.com/740629) and should not contain values that might
+ * change if the page is re-opened later.
  */
 
 /** @type {!LoadTimeData} */ var loadTimeData;