Make MemoryCache::contains() take a const Resource pointer.

This is needed by const-method clients such as "Resource::canDelete() const".

BUG=290193

Review URL: https://codereview.chromium.org/213193002

git-svn-id: svn://svn.chromium.org/blink/trunk@170154 bbb929c8-8fbe-4397-9dbb-9b2b20218538