DevTools: use timing information for "disk cached" resources.

"Disk Cached" resources bring valid timing info, so it should be shown to users.

Drive-by: get rid of "cache" property getters/setters.

BUG=410413

Review URL: https://codereview.chromium.org/637523002

git-svn-id: svn://svn.chromium.org/blink/trunk@183681 bbb929c8-8fbe-4397-9dbb-9b2b20218538