webdriver: Use take_screenshot() API in Take (Element) Screenshot (#39587)

WPT tests require us to reliably take screenshots when test pages are
fully loaded and testing is complete, and the WPT runner uses
[test-wait.js](a2f551eb2d/tests/wpt/tests/tools/wptrunner/wptrunner/executors/test-wait.js)
to do this in userland. when testing Servo with [--product
servo](a2f551eb2d/tests/wpt/tests/tools/wptrunner/wptrunner/executors/executorservo.py),
we use servoshell’s --output option, which backs that up with more
reliable waiting in Servo. but when testing Servo with [--product
servodriver](a2f551eb2d/tests/wpt/tests/tools/wptrunner/wptrunner/executors/executorservodriver.py),
we use the WebDriver Take Screenshot action, which currently takes the
screenshot immediately. we think this might be a source of regressions.

this patch makes the WebDriver actions Take Screenshot and Take Element
Screenshot use the same new WebView::take_screenshot() API as
servoshell’s --output option, such that those actions now wait for [a
variety of
conditions](a2f551eb2d/components/servo/webview.rs (L596-L602))
that may affect test output. it’s not clear if this is
[conformant](https://w3c.github.io/webdriver/#screen-capture), so we may
want to refine this to only wait when running tests at some point. other
changes:

- we remove the retry loop where we try to take a screenshot every
second for up to 30 seconds
- we send the result as a image::RgbaImage over crossbeam without shared
memory (it’s not cross-process)
- we now handle the zero-sized element case directly in the WebDriver
server

Testing: This should fix some flaky tests.
Fixes: #36715.
Fixes: (partially) #39180.
Fixes: (partially) #34683.

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/meta/webdriver/tests/classic/take_element_screenshot/scroll_into_view.py.ini

@@ -1,3 +0,0 @@
-[scroll_into_view.py]
-  [test_scroll_into_view]
-    expected: FAIL