Rename InputType::observeFeatureIfVisible to countUsageIfVisible.

The name `observeFeature' came from FeatureObserver, which was the old name of
UseCounter. We should synchronize with the current name `UseCounter.'

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/42993003

git-svn-id: svn://svn.chromium.org/blink/trunk@160523 bbb929c8-8fbe-4397-9dbb-9b2b20218538