Optimized computeDistance function in GestureRecognizer

Reviewers: lmoura, hugopl, setanta, Tullio, nick, zanoni, lacerda

Differential Revision: http://phabricator.coderev.indt.org/D3170
Makes MiniBrowser's handlePinch call WKViewGetContentPosition only once

Reviewers: luck, lmoura, Tullio, setanta, nick, zanoni, hugopl

Differential Revision: http://phabricator.coderev.indt.org/D3171