Disk Cache: Eliminate a race in one of the new tests created by r103323
Disk Cache: Eliminate a race in one of the new tests
created by r103323

BUG=73102
TEST=green memory tree
TBR=gavinp
Review URL: http://codereview.chromium.org/8088001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@103349 0039d316-1c4b-4281-b951-d872f2087c98

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