Change SkResourceCache to take a Visitor inside its find().

This simplifies the API/contract, in that there are not any exposed
lock/unlock scopes.

patch from issue 572573002

BUG=skia:
R=mtklein@google.com, danakj@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/567393002
5 files changed