Remove unused WebRuntimeFeature query methods

The embedder almost never needs to ask whether a feature of Blink is enabled
via the API, so only provide those query method where they're actually used
(just two of them).

There was already precedent for this in WebRuntimeFeatures, as not all features
had associated 'isXXXEnabled()' methods.

R=abarth

Review URL: https://codereview.chromium.org/148793005

git-svn-id: svn://svn.chromium.org/blink/trunk@166642 bbb929c8-8fbe-4397-9dbb-9b2b20218538