websql: Make storage::DatabaseConnections::connections_ non-mutable.

DatabaseConnections::connections_ is only mutable so that
DatabaseConnections::GetOpenDatabaseSize() can use std::map::operator[]
while being const.

This CL removes the mutable qualifier and replaces operator[] use in
GetOpenDatabaseSize() with find() and const_iterator, to make it easier
to reason about thread safety and invariants.

Change-Id: Ifb9df3733c099d9b686096f71156d0a4b2c6271a
Reviewed-on: https://chromium-review.googlesource.com/1137929Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575454}

storage/common/database/database_connections.cc

@@ -71,8 +71,12 @@ DatabaseConnections::RemoveConnections(const DatabaseConnections& connections) {
 int64_t DatabaseConnections::GetOpenDatabaseSize(
     const std::string& origin_identifier,
     const base::string16& database_name) const {
-  DCHECK(IsDatabaseOpened(origin_identifier, database_name));
-  return connections_[origin_identifier][database_name].second;
+  OriginConnections::const_iterator origin_it =
+      connections_.find(origin_identifier);
+  DCHECK(origin_it != connections_.end()) << "Database not opened";
+  DBConnections::const_iterator it = origin_it->second.find(database_name);
+  DCHECK(it != origin_it->second.end()) << "Database not opened";
+  return it->second.second;
 }
 
 void DatabaseConnections::SetOpenDatabaseSize(

storage/common/database/database_connections.h

@@ -38,7 +38,7 @@ class STORAGE_COMMON_EXPORT DatabaseConnections {
   std::vector<std::pair<std::string, base::string16>> RemoveConnections(
       const DatabaseConnections& connections);
 
-  // Database sizes can be kept only if IsDatabaseOpened returns true.
+  // Can be called only if IsDatabaseOpened would have returned true.
   int64_t GetOpenDatabaseSize(const std::string& origin_identifier,
                               const base::string16& database_name) const;
   void SetOpenDatabaseSize(const std::string& origin_identifier,
@@ -49,10 +49,12 @@ class STORAGE_COMMON_EXPORT DatabaseConnections {
   std::vector<std::pair<std::string, base::string16>> ListConnections() const;
 
  private:
-  // Mapping from name to <openCount, size>
-  typedef std::map<base::string16, std::pair<int, int64_t>> DBConnections;
-  typedef std::map<std::string, DBConnections> OriginConnections;
-  mutable OriginConnections connections_;  // mutable for GetOpenDatabaseSize
+  // Maps database names to (open count, database size).
+  using DBConnections = std::map<base::string16, std::pair<int, int64_t>>;
+  // Maps origin identifiers to DBConnections.
+  using OriginConnections = std::map<std::string, DBConnections>;
+
+  OriginConnections connections_;
 
   // Returns true if the last connection was removed.
   bool RemoveConnectionsHelper(const std::string& origin_identifier,