LibWeb: Set size of canvas used to take WebDriver screenshots explicitly

[ADKaster]

The default canvas size is 150x150 pixels. If the element or document we are trying to screenshot for the WebDriver is not at least that size, then we will create a canvas that is taller or larger than the actual element we are painting, resulting in a bunch of transparent pixels falling off the end.

This fixes 14 WPT css/CSS2/floats tests that we run in CI, and presumably a ton of other reftests in the WPT test suite.

[kalenikaliaksandr]

The default canvas size is 150x150 pixels

Spec says it is 300px x 150px https://html.spec.whatwg.org/multipage/canvas.html#attr-canvas-width

[ADKaster]

Spec says it is 300px x 150px html.spec.whatwg.org/multipage/canvas.html#attr-canvas-width

Fixed in the commit message and PR desc, thanks!