Fix incorrect assertion in <iframe srcdoc> attributes processing test

* "Changing" => "Setting", per https://github.com/web-platform-tests/wpt/pull/17437#discussion_r298800601.
* "load" event should fire after "srcdoc" attribute modification, per https://github.com/web-platform-tests/wpt/pull/17437#issuecomment-549563144.
diff --git a/html/semantics/embedded-content/the-iframe-element/srcdoc_process_attributes.html b/html/semantics/embedded-content/the-iframe-element/srcdoc_process_attributes.html
index 397c95a..0bd9f9b 100644
--- a/html/semantics/embedded-content/the-iframe-element/srcdoc_process_attributes.html
+++ b/html/semantics/embedded-content/the-iframe-element/srcdoc_process_attributes.html
@@ -20,7 +20,7 @@
     assert_equals(iframe.contentDocument.body.textContent, "src");
 
     iframe.onload = t.step_func_done(function() {
-      assert_false(isAdded);
+      assert_true(isAdded);
       assert_equals(iframe.contentDocument.location.href, "about:srcdoc");
       assert_equals(iframe.contentDocument.body.textContent, "srcdoc");
     });
@@ -41,7 +41,7 @@
     assert_equals(iframe.contentDocument.body.textContent, "old");
 
     iframe.onload = t.step_func_done(function() {
-      assert_false(isChanged);
+      assert_true(isChanged);
       assert_equals(iframe.contentDocument.location.href, "about:srcdoc");
       assert_equals(iframe.contentDocument.body.textContent, "new");
     });
@@ -51,7 +51,7 @@
   });
 
   document.body.appendChild(iframe);
-}, "Changing `srcdoc` (via property) triggers attributes processing");
+}, "Setting `srcdoc` (via property) triggers attributes processing");
 
 async_test(function(t) {
   var iframe = createIFrameWithBlobSrc();
@@ -62,7 +62,7 @@
     assert_equals(iframe.contentDocument.body.textContent, "srcdoc");
 
     iframe.onload = t.step_func_done(function() {
-      assert_false(isRemoved);
+      assert_true(isRemoved);
       assert_equals(iframe.contentDocument.location.protocol, "blob:");
       assert_equals(iframe.contentDocument.body.textContent, "src");
     });