Fix comment to match implementation.

NOTRY=true
BUG=None

Review URL: https://codereview.chromium.org/504443003

Cr-Commit-Position: refs/heads/master@{#291694}

content/browser/geolocation/geolocation_provider_impl.h

@@ -37,7 +37,7 @@ class CONTENT_EXPORT GeolocationProviderImpl
 
   // Gets a pointer to the singleton instance of the location relayer, which
   // is in turn bound to the browser's global context objects. This must only be
-  // called on the IO thread so that the GeolocationProviderImpl is always
+  // called on the UI thread so that the GeolocationProviderImpl is always
   // instantiated on the same thread. Ownership is NOT returned.
   static GeolocationProviderImpl* GetInstance();