OffScreen - Add CaptureScreenshotAsync, GetContentSizeAsync and ResizeAsync

- Simply taking full page screenshots
- Added new methods
- CaptureScreenshotAsync uses Devtools protocol and allows for specifying image format and quality (if jpg)

Cannot use clip param as https://bitbucket.org/chromiumembedded/cef/issues/3103/offscreen-capture-screenshot-with-devtools