[wptrunner] Create shared WebDriverPrintRefTestExecutor implementation

[jonathan-j-lee]

The WebDriver spec defines an endpoint to print a page as a PDF, so it seems possible to run print-reftests the same way between WebDriver-compliant browsers.

This will also let us replace a direct nonstandard call to CDP in Chromium's print-reftest executor:

wpt/tools/wptrunner/wptrunner/executors/executorchrome.py

Lines 166 to 179 in 17a5786

	def render_as_pdf(self, width, height):
	margin = 0.5
	params = {
	# Chrome accepts dimensions in inches; we are using cm
	"paperWidth": width / 2.54,
	"paperHeight": height / 2.54,
	"marginLeft": margin,
	"marginRight": margin,
	"marginTop": margin,
	"marginBottom": margin,
	"shrinkToFit": False,
	"printBackground": True,
	}
	return self.parent.cdp.execute_cdp_command("Page.printToPDF", params)["data"]

... which mostly lines up with WebDriver's /session/<id>/print already.