Disk cache: Re-enable DiskCacheEntryTest.CancelSparseIO
Disk cache: Re-enable DiskCacheEntryTest.CancelSparseIO

I still don't know why it failed, but the only difference
that I could see in this test between xp and vista is that
xp accepts a lot of small IOs before returning pending, so
this change results in similar behavior in both platforms
and a faster run on xp.

BUG=23862
TEST=none


Review URL: http://codereview.chromium.org/273024

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@28921 0039d316-1c4b-4281-b951-d872f2087c98

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