Fix a crash in the FileSystem API when the app window closes during a call.

Previously, closing the app window while a file is being checked for
write access caused a crash. This adds an early exit to prevent that
crash.

BUG=282727

Review URL: https://chromiumcodereview.appspot.com/23629033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222224 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/extensions/api/file_system/file_system_api.cc

@@ -339,6 +339,8 @@ void FileSystemEntryFunction::CheckWritableFiles(
 void FileSystemEntryFunction::RegisterFileSystemsAndSendResponse(
     const std::vector<base::FilePath>& paths) {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  if (!render_view_host_)
+    return;
 
   CreateResponse();
   for (std::vector<base::FilePath>::const_iterator it = paths.begin();