Add a value for ent-to-end reloads in Cache policy

This CL adds a value for end-to-end reload in the cache policy. This will allow
WebURLLoader to set the right load flag (net::LOAD_BYPASSING_CACHE) when
sending the resource request to the browser, without blink having to set any
HTPP cache header ("Pragma: no-cache", "Cache-Control: no-cache").
https://chromiumcodereview.appspot.com/308083009/ is the Chrome CL that handles the new value.

BUG=376025

Review URL: https://codereview.chromium.org/309693002

git-svn-id: svn://svn.chromium.org/blink/trunk@175386 bbb929c8-8fbe-4397-9dbb-9b2b20218538