Resource cache: unlock Resource when the last client goes away

When the last client of a resource goes away, and it does not have any handle
anymore, this CL unlocks the resource in order to decrease the memory
consumption of the MemoryCache. This CL also adds the possibility to unlock FontResources.

BUG=290193

Review URL: https://codereview.chromium.org/177833002

git-svn-id: svn://svn.chromium.org/blink/trunk@169112 bbb929c8-8fbe-4397-9dbb-9b2b20218538