Split out  const/non-const variants of Resource::resourceRequest().

This patch follows the general convention of *::resourceRequest() and
*::mutableResourceRequest() from various other classes which use
ResourceRequest objects (FetchRequest, NavigationAction, etc).

As a drive-by, it also consts Resource::lastResourceRequest(), which was
already returning a const reference.

NOTRY=true

Review URL: https://codereview.chromium.org/424083002

git-svn-id: svn://svn.chromium.org/blink/trunk@179143 bbb929c8-8fbe-4397-9dbb-9b2b20218538