[Tizen] Fix memory leaks found in xwalk_tizen_user.cc

Memory leaks appear when xwalk_backend runs correctly.
This patch adds freeing buffer after usage.

BUG=XWALK-3206
This shouldn't be here. We follow [this rule here](https://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Names_and_Order_of_Includes) when ordering the includes. Since `tzplatform_config.h` is not a standard C or C++ header, it should come after this block and before `scoped_ptr.h`, which should be in a block of its own (I consider it "part of our project").