Rename FeatureObserver to UseCounter to make clear that all it stores are usage counts.

I also added a comment to the top of UseCounter.h which references Chrome's privacy policy and explains when use-count data is ever submitted to Google.

Review URL: https://codereview.chromium.org/14301003

git-svn-id: svn://svn.chromium.org/blink/trunk@148497 bbb929c8-8fbe-4397-9dbb-9b2b20218538
36 files changed