Fix case where an unmounted partition is accessed during tracing on Windows.
Fix case where an unmounted partition is accessed during tracing on Windows.

R=nsylvain@chromium.org
BUG=98834
TEST=


Review URL: http://codereview.chromium.org/10221022

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@134404 0039d316-1c4b-4281-b951-d872f2087c98

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