Cleanup: Return error status as bool in FileSystemUsageCache functions.

Current impl returns error status and its result together.
Before adding further change, this should be separated to return value and parameter.

BUG=220029
TEST='FileSystemUsageCacheTest.*'

Review URL: https://chromiumcodereview.appspot.com/13922008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@194125 0039d316-1c4b-4281-b951-d872f2087c98
5 files changed