Remove unused UseCounter features

These are the reviews in which the enum values became unused:

SVGInstanceRoot:
https://codereview.chromium.org/272523002

PromiseConstructor/PromiseCast/PromiseReject/PromiseResolve:
https://codereview.chromium.org/293933002

TextAutosizingLayout:
https://codereview.chromium.org/346603005

PrefixedElementRequestPointerLock/PrefixedDocumentExitPointerLock/PrefixedDocumentPointerLockElement:
https://codereview.chromium.org/437443005

PrefixedConvertPointFromPageToNode/PrefixedConvertPointFromNodeToPage:
https://codereview.chromium.org/428533003

MixedContentTextTrack:
https://codereview.chromium.org/348853009

Review URL: https://codereview.chromium.org/442323002

git-svn-id: svn://svn.chromium.org/blink/trunk@179627 bbb929c8-8fbe-4397-9dbb-9b2b20218538