Fix leak of DictionaryValue in ApplicationStorageImpl::GetInstalledApplications()

This patch fixes the leak of DictionaryValue returned by JSONFileValueSerializer::Deserialize().

valgrind log:
==9706== 2,576 (48 direct, 2,528 indirect) bytes in 1 blocks are definitely lost in loss record 4,268 of 4,344
==9706==    at 0x95173FC: operator new(unsigned long) (vg_replace_malloc.c:1140)
.....
==9706==    by 0x5026B9: JSONStringValueSerializer::Deserialize(int*, std::string*) (json_string_value_serializer.cc:47)
==9706==    by 0x3532E25: xwalk::application::ApplicationStorageImpl::GetInstalledApplications(std::map<std::string, scoped_refptr<xwalk::application::ApplicationData>, std::less<std::string>, std::allocator<std::pair<std::string const, scoped_refptr<xwalk::application::ApplicationData> > > >&) (application_storage_impl.cc:201)