Bug 1561227 Part 8: Update expectations for existing tests. r=botond

The change to await snapshotWindow is something that should have been
done in Bug 1573254.

Differential Revision: https://phabricator.services.mozilla.com/D47510

--HG--
extra : moz-landing-system : lando

devtools/client/responsive/test/browser/browser_viewport_resizing_scrollbar.js

@@ -87,7 +87,7 @@ addRDMTask(TEST_URL, async function({ ui, manager }) {
 
     // Get to the initial size and snapshot the window.
     await setViewportSize(ui, manager, 300, 600);
-    const initialSnapshot = snapshotWindow(browser.contentWindow);
+    const initialSnapshot = await snapshotWindow(browser.contentWindow);
 
     // Move to the rotated size.
     await setViewportSize(ui, manager, 600, 300);
@@ -99,7 +99,7 @@ addRDMTask(TEST_URL, async function({ ui, manager }) {
 
     // Go back to the initial size and take another snapshot.
     await setViewportSize(ui, manager, 300, 600);
-    const finalSnapshot = snapshotWindow(browser.contentWindow);
+    const finalSnapshot = await snapshotWindow(browser.contentWindow);
 
     const result = compareSnapshots(initialSnapshot, finalSnapshot, true);
     is(result[2], result[1], "Window snapshots should match.");