use size_t instead of unsigned long
use size_t instead of unsigned long

Summary:
Use ```size_t```instead of ```unsinged long``` as this is the "offical" return type of ```strlen```. Is VS13 ```size_t``` is defined as ```unsigned long long``` which leads to a compiler warning.
Closes https://github.com/facebook/css-layout/pull/254

Reviewed By: emilsjolander

Differential Revision: D4199327

Pulled By: splhack

fbshipit-source-id: 5e1a91f282bf776e4d9f5806e6467dfe36c7a633

yihuang/css-layout,rmarinho/yoga,facebook/yoga,facebook/yoga,facebook/yoga,facebook/yoga,yihuang/css-layout,yihuang/css-layout,rmarinho/yoga,facebook/css-layout,rmarinho/yoga,facebook/yoga,facebook/css-layout,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,rmarinho/yoga,facebook/css-layout,yihuang/css-layout,facebook/css-layout,yihuang/css-layout,facebook/css-layout,facebook/yoga,facebook/css-layout,rmarinho/yoga,rmarinho/yoga,rmarinho/yoga,rmarinho/yoga,facebook/yoga,facebook/css-layout,yihuang/css-layout,facebook/yoga,yihuang/css-layout,rmarinho/yoga,yihuang/css-layout,rmarinho/yoga,yihuang/css-layout,yihuang/css-layout