Add a version of WebApplicationCacheHost::willStartMainResourceRequest that doesn't depend on WebFrame

This is in preparation for removing WebApplicationCacheHost's dependency
public/web, allowing it to move to public/platform (and thus allow it to
be referenced from core).

BUG=343272

Review URL: https://codereview.chromium.org/161303003

git-svn-id: svn://svn.chromium.org/blink/trunk@167055 bbb929c8-8fbe-4397-9dbb-9b2b20218538