DevTools: show HTTP headers of cached resources in network panel.

Screenshot: http://picpaste.com/RaGXNCAZ.png

BUG=91276

Review URL: https://codereview.chromium.org/300913002

git-svn-id: svn://svn.chromium.org/blink/trunk@175535 bbb929c8-8fbe-4397-9dbb-9b2b20218538