Add explicit destructor to DatabaseBackend.h.

This class was missed in https://codereview.chromium.org/412483007/, see that
CL's description for the motivation.

I filed http://llvm.org/20452 about the diagnostic for this not being as clear
as it could be.

BUG=82385
R=rnk@chromium.org
TBR=michaeln

Review URL: https://codereview.chromium.org/419853002

git-svn-id: svn://svn.chromium.org/blink/trunk@178945 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/webdatabase/DatabaseBackend.cpp

@@ -47,6 +47,10 @@ DatabaseBackend::DatabaseBackend(DatabaseContext* databaseContext, const String&
 {
 }
 
+DatabaseBackend::~DatabaseBackend()
+{
+}
+
 void DatabaseBackend::trace(Visitor* visitor)
 {
     visitor->trace(m_transactionQueue);

third_party/WebKit/Source/modules/webdatabase/DatabaseBackend.h

@@ -49,6 +49,7 @@ class SQLTransactionCoordinator;
 class DatabaseBackend : public DatabaseBackendBase {
 public:
     DatabaseBackend(DatabaseContext*, const String& name, const String& expectedVersion, const String& displayName, unsigned long estimatedSize);
+    virtual ~DatabaseBackend();
     virtual void trace(Visitor*) OVERRIDE;
 
     virtual bool openAndVerifyVersion(bool setVersionInNewDatabase, DatabaseError&, String& errorMessage) OVERRIDE FINAL;