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Also change valgrind_tests.py to use cygpath to give DrMemory Windows paths
when running with Cygwin Python.

The original change was made by Reid Kleckner and reviewed at
http://codereview.chromium.org/8505028/

TBR=glider
Review URL: http://codereview.chromium.org/8636008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@111129 0039d316-1c4b-4281-b951-d872f2087c98