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: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@29720 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser