AppCache: Fix AppCacheService threading documentation.

Before this CL, AppCacheService's documentation claimed that its methods
should be used on the IO thread. However, the method implementations in
AppCacheServiceImpl DCHECK that they're used on the UI thread.

This CL updates the documentation to reflect reality.

Change-Id: I09f1cdbdb27c0e50d55f2ad043b0612eae5287ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513351
Auto-Submit: Victor Costan <pwnall@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823038}

content/public/browser/appcache_service.h

@@ -37,7 +37,7 @@ struct CONTENT_EXPORT AppCacheInfoCollection
 };
 
 // Exposes a limited interface to the AppCacheService.
-// Call these methods only on the IO thread.
+// Call these methods only on the UI thread.
 class CONTENT_EXPORT AppCacheService {
  public:
   // Populates 'collection' with info about all of the appcaches stored