[ios] Safely handle non-existent plists in OmniboxGeolocationConfig.

ios/chrome/browser/geolocation/omnibox_geolocation_config.mm

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/geolocation/omnibox_geolocation_config.h"
 
 #include <set>
 #include <string>
 
 #include "base/logging.h"
@@ skipping 49 matching lines @@
   return NO;
 }
 
 #pragma mark - Private
 
 - (void)loadConfigFile:(NSString*)plistFileName {
   NSString* path = [[NSBundle mainBundle] pathForResource:plistFileName
                                                    ofType:@"plist"
                                               inDirectory:@"gm-config/ANY"];
   NSDictionary* configData = [NSDictionary dictionaryWithContentsOfFile:path];
+  if (!configData) {
+    // The plist is not packaged with Chromium builds.  This is not an error, so
+    // simply return early, since no domains are eligible for geolocation.
+    return;
+  }
+
   NSArray* eligibleDomains = base::mac::ObjCCastStrict<NSArray>(
       [configData objectForKey:kEligibleDomainsKey]);
   if (eligibleDomains) {
     for (id item in eligibleDomains) {
       NSString* domain = base::mac::ObjCCastStrict<NSString>(item);
       if ([domain length]) {
         _eligibleDomains.insert(
             base::SysNSStringToUTF8([domain lowercaseString]));
       }
     }
   }
+  // Make sure that if a plist exists, it contains at least one eligible domain.
   DCHECK(!_eligibleDomains.empty());
 }
 
 @end