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

This CL adds an early return rather than DCHECKing when the plist is missing.

BUG=666375

Review-Url: https://codereview.chromium.org/2551423002
Cr-Commit-Position: refs/heads/master@{#438719}

ios/chrome/browser/geolocation/omnibox_geolocation_config.mm

@@ -67,6 +67,12 @@ NSString* const kEligibleDomainsKey = @"EligibleDomains";
                                                    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) {
@@ -78,6 +84,7 @@ NSString* const kEligibleDomainsKey = @"EligibleDomains";
       }
     }
   }
+  // Make sure that if a plist exists, it contains at least one eligible domain.
   DCHECK(!_eligibleDomains.empty());
 }