bug 1385706: marionette: fix WebDriver:TakeScreenshot to use viewport bounds; r=ato

The WebDriver:TakeScreenshot command relied on the document element's
clientWidth/clientHeight, but should according to the WebDriver
specification use the viewport's dimensions.

Thanks-to: JinaJita <jitajina@gmail.com>

testing/marionette/capture.js

@@ -72,14 +72,12 @@ capture.element = function(node, highlights = []) {
  *     The canvas element where the viewport has been painted on.
  */
 capture.viewport = function(win, highlights = []) {
-  let rootNode = win.document.documentElement;
-
   return capture.canvas(
       win,
       win.pageXOffset,
       win.pageYOffset,
-      rootNode.clientWidth,
-      rootNode.clientHeight,
+      win.innerWidth,
+      win.innerHeight,
       {highlights});
 };
 

testing/marionette/harness/marionette_harness/tests/unit/test_screenshot.py

@@ -67,10 +67,7 @@ class ScreenCaptureTestCase(MarionetteTestCase):
 
     @property
     def viewport_dimensions(self):
-        return self.marionette.execute_script("""
-            return [arguments[0].clientWidth,
-                    arguments[0].clientHeight];
-            """, script_args=[self.document_element])
+        return self.marionette.execute_script("return [window.innerWidth, window.innerHeight];")
 
     def assert_png(self, screenshot):
         """Test that screenshot is a Base64 encoded PNG file."""

testing/web-platform/tests/webdriver/tests/take_screenshot/__init__.py

@@ -1,6 +1,2 @@
 def document_dimensions(session):
-    return tuple(session.execute_script("""
-        let devicePixelRatio = window.devicePixelRatio;
-        let rect = document.documentElement.getBoundingClientRect();
-        return [Math.floor(rect.width * devicePixelRatio), Math.floor(rect.height * devicePixelRatio)];
-        """))
+    return tuple(session.execute_script("return [window.innerWidth, window.innerHeight];"))