Fix compilation error in mojo/shell/test_package_manager.h

Compilation error in http://crrev.com/1352663002

FAILED: /b/build/goma/gomacc ../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/mojo/shell/mojo_shell_unittests/test_package_manager.o.d ...omit... -c ../../mojo/shell/test_package_manager.cc -o obj/mojo/shell/mojo_shell_unittests/test_package_manager.o
In file included from ../../mojo/shell/test_package_manager.cc:5:
../../mojo/shell/test_package_manager.h:23:32: error: [chromium-style] Overriding method must be marked with 'override' or 'final'.
  virtual ~TestPackageManager();
                               ^
                                override
1 error generated.

BUG=n/a
TBR=yzshen@chromium.org,ben@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Committed: https://crrev.com/c5e4e66b7b63f247391730735a4fcd4bfca41ae4
Cr-Commit-Position: refs/heads/master@{#349358}

[yosin_UTC9, 2015-09-17 07:02:27]

The CQ bit was checked by yosin@chromium.org

[commit-bot: I haz the power, 2015-09-17 07:02:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1351833002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1351833002/1

[commit-bot: I haz the power, 2015-09-17 07:03:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-17 07:04:15]

Patchset 1 (id:??) landed as
https://crrev.com/c5e4e66b7b63f247391730735a4fcd4bfca41ae4
Cr-Commit-Position: refs/heads/master@{#349358}