WebDriver+Browser: Implement GET /session/{id}/screenshot

This doesn't follow the spec to a tee. Our OutOfProcessWebView already
has a bitmap that can be used as the window screenshot. Therefore, we
can bypass the steps that assume we need to access the window's frame
buffer in-flight.

We also don't create an HTMLCanvasElement. We would need a Document in
the WebDriver process to do so. Instead, we can still run the encoding
steps exactly as-is using the screenshot bitmap.

Userland/Services/WebDriver/Session.cpp

@@ -10,11 +10,14 @@
 #include "Session.h"
 #include "BrowserConnection.h"
 #include "Client.h"
+#include <AK/Base64.h>
 #include <AK/NumericLimits.h>
 #include <AK/Time.h>
+#include <AK/URL.h>
 #include <LibCore/LocalServer.h>
 #include <LibCore/Stream.h>
 #include <LibCore/System.h>
+#include <LibGfx/PNGWriter.h>
 #include <LibGfx/Point.h>
 #include <LibGfx/Rect.h>
 #include <LibGfx/Size.h>
@@ -1123,4 +1126,52 @@ ErrorOr<JsonValue, WebDriverError> Session::delete_all_cookies()
     return JsonValue();
 }
 
+// https://w3c.github.io/webdriver/#dfn-encoding-a-canvas-as-base64
+static ErrorOr<String, WebDriverError> encode_bitmap_as_canvas_element(Gfx::Bitmap const& bitmap)
+{
+    // FIXME: 1. If the canvas element’s bitmap’s origin-clean flag is set to false, return error with error code unable to capture screen.
+
+    // 2. If the canvas element’s bitmap has no pixels (i.e. either its horizontal dimension or vertical dimension is zero) then return error with error code unable to capture screen.
+    if (bitmap.width() == 0 || bitmap.height() == 0)
+        return WebDriverError::from_code(ErrorCode::UnableToCaptureScreen, "Captured screenshot is empty"sv);
+
+    // 3. Let file be a serialization of the canvas element’s bitmap as a file, using "image/png" as an argument.
+    auto file = Gfx::PNGWriter::encode(bitmap);
+
+    // 4. Let data url be a data: URL representing file. [RFC2397]
+    auto data_url = AK::URL::create_with_data("image/png"sv, encode_base64(file), true).to_string();
+
+    // 5. Let index be the index of "," in data url.
+    auto index = data_url.find(',');
+    VERIFY(index.has_value());
+
+    // 6. Let encoded string be a substring of data url using (index + 1) as the start argument.
+    auto encoded_string = data_url.substring(*index + 1);
+
+    // 7. Return success with data encoded string.
+    return encoded_string;
+}
+
+// 17.1 Take Screenshot, https://w3c.github.io/webdriver/#take-screenshot
+ErrorOr<JsonValue, WebDriverError> Session::take_screenshot()
+{
+    // 1. If the current top-level browsing context is no longer open, return error with error code no such window.
+    TRY(check_for_open_top_level_browsing_context_or_return_error());
+
+    // 2. When the user agent is next to run the animation frame callbacks:
+    //     a. Let root rect be the current top-level browsing context’s document element’s rectangle.
+    //     b. Let screenshot result be the result of trying to call draw a bounding box from the framebuffer, given root rect as an argument.
+    auto screenshot = m_browser_connection->take_screenshot();
+    if (!screenshot.is_valid())
+        return WebDriverError::from_code(ErrorCode::UnableToCaptureScreen, "Unable to capture screenshot"sv);
+
+    //     c. Let canvas be a canvas element of screenshot result’s data.
+    //     d. Let encoding result be the result of trying encoding a canvas as Base64 canvas.
+    //     e. Let encoded string be encoding result’s data.
+    auto encoded_string = TRY(encode_bitmap_as_canvas_element(*screenshot.bitmap()));
+
+    // 3. Return success with data encoded string.
+    return encoded_string;
+}
+
 }