It's currently an inefficient thread implementation. Interestingly
enough, callback support is significantly faster on cygwin than on
native python.

Writing an efficient implementation is punted for a later change,
one per implementation.

Stops using a temporary file since it's not necessary anymore.

The goal is to reduce the number of places where a similar paradigm
is used by having a canonical generic implementation.

R=dpranke@chromium.org
BUG=
TEST=Tested manually on Windows, cygwin, linux, OSX 10.6


Review URL: http://codereview.chromium.org/8374026

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@109239 0039d316-1c4b-4281-b951-d872f2087c98