Fix automated_ui_testing for XMLWriter

Fix automated_ui_testing for XMLWriter

There was an error in InitXMLReader where it tries to delete the bufffer_
member, 
setting it to null will make it work as mulhern stated.

   \src\chrome\Debug>automated_ui_tests.exe
   Started crash_service.exe so you know if a test crashes!
   [==========] Running 1 test from 1 test case.
   [----------] Global test environment set-up.
   [----------] 1 test from AutomatedUITest
   [ RUN      ] AutomatedUITest.TheOneAndOnlyTest
   .\automated_ui_tests.cc(167): error: Value of: InitXMLReader()
   Actual: false
   Expected: true
   Error initializing XMLReader

Then it crashes.

TEST=Ran the test again, doesn't crash.

BUG=9625
http://crbug.com/9625

Patch by Mohamed Mansour <m0.interactive@gmail.com>; in
http://codereview.chromium.org/63001