Disk cache: Fix a race accessing a file's reference counter from a worker thread.
Disk cache: Fix a race accessing a file's reference counter
from a worker thread.

BUG=24307
TEST=current unit tests / valgrind

Review URL: http://codereview.chromium.org/309001

git-svn-id: http://src.chromium.org/svn/trunk/src@29720 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: f39b51fb5456a238828cbb29246b8e9eeffd4cb7
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser