[sql] Change DoesStuffExist() to work with ScopedErrorIgnorer.

sql/connection.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SQL_CONNECTION_H_
 #define SQL_CONNECTION_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 343 matching lines @@
 
   // Returns a non-cached statement for the given SQL. Use this for SQL that
   // is only executed once or only rarely (there is overhead associated with
   // keeping a statement cached).
   //
   // See GetCachedStatement above for examples and error information.
   scoped_refptr<StatementRef> GetUniqueStatement(const char* sql);
 
   // Info querying -------------------------------------------------------------
 
-  // Returns true if the given table exists.
+  // Returns true if the given table (or index) exists.  Instead of
+  // test-then-create, callers should almost always prefer "CREATE TABLE IF NOT
+  // EXISTS" or "CREATE INDEX IF NOT EXISTS".
   bool DoesTableExist(const char* table_name) const;
-
-  // Returns true if the given index exists.
   bool DoesIndexExist(const char* index_name) const;
 
   // Returns true if a column with the given name exists in the given table.
   bool DoesColumnExist(const char* table_name, const char* column_name) const;
 
   // Returns sqlite's internal ID for the last inserted row. Valid only
   // immediately after an insert.
   int64 GetLastInsertRowId() const;
 
   // Returns sqlite's count of the number of rows modified by the last
@@ skipping 214 matching lines @@
 
   // Tag for auxiliary histograms.
   std::string histogram_tag_;
 
   DISALLOW_COPY_AND_ASSIGN(Connection);
 };
 
 }  // namespace sql
 
 #endif  // SQL_CONNECTION_H_