Oilpan: Attach/detach the main thread in RunAllTests.cpp

After r171258, we need to explicitly attach/detach the main thread.
This CL does the attach/detach in RunAllTests.cpp.

TBR=ager@chromium.org

Review URL: https://codereview.chromium.org/232083008

git-svn-id: svn://svn.chromium.org/blink/trunk@171261 bbb929c8-8fbe-4397-9dbb-9b2b20218538