Using leveldb_chrome::PossiblyValidDB in GCMStoreImpl.

Replaced DatabaseExists with new leveldb_chrome::PossiblyValidDB.

Bug: None
Change-Id: Iecb181588355c3f59953230ad0946f8e531c8a3f
Reviewed-on: https://chromium-review.googlesource.com/1090998Reviewed-by: Dmitry Titov <dimich@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567480}

google_apis/gcm/engine/gcm_store_impl.cc

@@ -26,6 +26,7 @@
 #include "google_apis/gcm/base/mcs_util.h"
 #include "google_apis/gcm/protocol/mcs.pb.h"
 #include "third_party/leveldatabase/env_chromium.h"
+#include "third_party/leveldatabase/leveldb_chrome.h"
 #include "third_party/leveldatabase/src/include/leveldb/db.h"
 #include "third_party/leveldatabase/src/include/leveldb/write_batch.h"
 
@@ -176,13 +177,6 @@ leveldb::Slice MakeSlice(const base::StringPiece& s) {
   return leveldb::Slice(s.begin(), s.size());
 }
 
-bool DatabaseExists(const base::FilePath& path) {
-  // It's not enough to check that the directory exists, since DestroyDB
-  // sometimes leaves behind an empty directory
-  // (https://github.com/google/leveldb/issues/215).
-  return base::PathExists(path.Append(FILE_PATH_LITERAL("CURRENT")));
-}
-
 }  // namespace
 
 class GCMStoreImpl::Backend
@@ -292,7 +286,8 @@ LoadStatus GCMStoreImpl::Backend::OpenStoreAndLoadData(StoreOpenMode open_mode,
 
   // Checks if the store exists or not. Opening a db with create_if_missing
   // not set will still create a new directory if the store does not exist.
-  if (open_mode == DO_NOT_CREATE && !DatabaseExists(path_)) {
+  if (open_mode == DO_NOT_CREATE &&
+      !leveldb_chrome::PossiblyValidDB(path_, leveldb::Env::Default())) {
     DVLOG(2) << "Database " << path_.value() << " does not exist";
     return STORE_DOES_NOT_EXIST;
   }