Add a CHECK failure for when the resource bundle loading fails.

This matches the CHECK in chrome/app/chrome_main_delegate.cc
with the same message.

These errors are not recoverable, and currently result in a CHECK
being hit later in VariationsService::EnsureLocaleEquals(), which
is trying to guard for locale mismatch issues.

Also expands header comments to mention the empty string return
value being the failure case.

See ResourceBundle::LoadLocaleResources() implementation for the
failure details (i.e. if empty string was returned, we either
didn't load the pack file at all, or got an error loading it).

Bug: 1022934
Change-Id: I4cc670cf669ed824c3b2073586941931fd38e364
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141010Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758991}

chrome/browser/chrome_resource_bundle_helper.cc

@@ -87,8 +87,8 @@ std::string InitResourceBundleAndDetermineLocale(PrefService* local_state,
   // method InitSharedInstance is ignored.
   std::string actual_locale = ui::ResourceBundle::InitSharedInstanceWithLocale(
       preferred_locale, nullptr, ui::ResourceBundle::LOAD_COMMON_RESOURCES);
-  if (actual_locale.empty())
-    return actual_locale;
+  CHECK(!actual_locale.empty())
+      << "Locale could not be found for " << preferred_locale;
 
   // First run prefs needs data from the ResourceBundle, so load it now.
   {

ui/base/resource/resource_bundle.h

@@ -120,7 +120,8 @@ class UI_BASE_EXPORT ResourceBundle {
   };
 
   // Initialize the ResourceBundle for this process. Does not take ownership of
-  // the |delegate| value. Returns the language selected.
+  // the |delegate| value. Returns the language selected or an empty string if
+  // initialization failed (e.g. resource bundle not found or corrupted).
   // NOTE: Mac ignores this and always loads up resources for the language
   // defined by the Cocoa UI (i.e., NSBundle does the language work).
   //
@@ -194,11 +195,12 @@ class UI_BASE_EXPORT ResourceBundle {
                                    ScaleFactor scale_factor);
 
   // Changes the locale for an already-initialized ResourceBundle, returning the
-  // name of the newly-loaded locale.  Future calls to get strings will return
-  // the strings for this new locale.  This has no effect on existing or future
-  // image resources.  |locale_resources_data_| is protected by a lock for the
-  // duration of the swap, as GetLocalizedString() may be concurrently invoked
-  // on another thread.
+  // name of the newly-loaded locale, or an empty string if initialization
+  // failed (e.g. resource bundle not found or corrupted). Future calls to get
+  // strings will return the strings for this new locale. This has no effect on
+  // existing or future image resources. |locale_resources_data_| is protected
+  // by a lock for the duration of the swap, as GetLocalizedString() may be
+  // concurrently invoked on another thread.
   std::string ReloadLocaleResources(const std::string& pref_locale);
 
   // Gets image with the specified resource_id from the current module data.