Disk cache: Increase the maximum number of threads on the cache pool to 20.

Speculative fix to see if there's any perf change.

BUG=325311
TEST=none
R=willchan@chromium.org

Review URL: https://codereview.chromium.org/109363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239384 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239384