webdriver: add screenshot test utilities

This adds some test utilities to the WebDriver WPT tests for dealing
with screenshots and document dimensions.

Depends on D6886

Differential Revision: https://phabricator.services.mozilla.com/D6887

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1431148
gecko-commit: e30ce73ca4f120f38d619c1b626ff9f91f48a2f4
gecko-integration-branch: autoland
gecko-reviewers: whimboo

webdriver/tests/support/asserts.py

@@ -1,3 +1,6 @@
+import base64
+import imghdr
+
 from webdriver import Element, NoSuchAlertException, WebDriverException
 
 
@@ -189,3 +192,10 @@ def assert_move_to_coordinates(point, target, events):
             assert e["pageX"] == point["x"]
             assert e["pageY"] == point["y"]
             assert e["target"] == target
+
+
+def assert_png(screenshot):
+    """Test that screenshot is a Base64 encoded PNG file."""
+    image = base64.decodestring(screenshot)
+    mime_type = imghdr.what("", image)
+    assert mime_type == "png", "Expected image to be PNG, but it was {}".format(mime_type)

webdriver/tests/support/helpers.py

@@ -117,3 +117,11 @@ def is_element_in_viewport(session, element):
         return !(rect.right < 0 || rect.bottom < 0 ||
             rect.left > viewport.width || rect.top > viewport.height)
     """, args=(element,))
+
+
+def document_dimensions(session):
+    return tuple(session.execute_script("""
+        let {devicePixelRatio} = window;
+        let {width, height} = document.documentElement.getBoundingClientRect();
+        return [width * devicePixelRatio, height * devicePixelRatio];
+        """))

webdriver/tests/support/image.py

@@ -0,0 +1,11 @@
+import base64
+import struct
+
+from tests.support.asserts import assert_png
+
+
+def png_dimensions(screenshot):
+    assert_png(screenshot)
+    image = base64.decodestring(screenshot)
+    width, height = struct.unpack(">LL", image[16:24])
+    return int(width), int(height)