Font metrics, version 1

Spec : http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-textmetrics-width

BUG=277215

Review URL: https://codereview.chromium.org/141433026

git-svn-id: svn://svn.chromium.org/blink/trunk@168737 bbb929c8-8fbe-4397-9dbb-9b2b20218538