Modify WPTs to check for standards mode documents

The [1] PR will change the standard so that media and
text documents are in standard (no quirks) mode. This
CL updates the tests accordingly.

Chromium already made this change in code in Sept 2020
(see CLs on crbug.com/1131185) and found no compat
issues.

[1] whatwg/html#6745

Bug: 1131185
Change-Id: Ie5f5ae5eb802008bf27794c152620bdcc9948ba2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2945338
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#889933}

html/browsers/browsing-the-web/read-media/pageload-image.html

@@ -14,6 +14,7 @@
   function frameLoaded() {
     var testframe = document.getElementById('testframe');
     assert_equals(testframe.contentDocument.contentType, "image/png");
+    assert_equals(testframe.contentDocument.compatMode, "CSS1Compat", "Media documents should be in standards mode");
     var testframeChildren = testframe.contentDocument.body.childNodes;
     assert_equals(testframeChildren.length, 1, "Body of image document has 1 child");
     assert_equals(testframeChildren[0].nodeName, "IMG", "Only child of body must be an <img> element");

html/browsers/browsing-the-web/read-media/pageload-video.html

@@ -15,6 +15,7 @@
   var contentType = getMediaContentType(url);
   testframe.onload = this.step_func_done(function() {
     assert_equals(testframe.contentDocument.contentType, contentType);
+    assert_equals(testframe.contentDocument.compatMode, "CSS1Compat", "Media documents should be in standards mode");
     var testframeChildren = testframe.contentDocument.body.childNodes;
     assert_equals(testframeChildren.length, 1, "Body of image document has 1 child");
     assert_equals(testframeChildren[0].nodeName, "VIDEO", "Only child of body must be an <video> element");

html/browsers/browsing-the-web/read-text/load-text-plain.html

@@ -13,7 +13,7 @@
 iframe.onload = function(e) {
   var doc = iframe.contentDocument;
   t.step(function() {
-    assert_equals(doc.compatMode, "BackCompat");
+    assert_equals(doc.compatMode, "CSS1Compat");
     assert_equals(doc.contentType, "text/plain");
     assert_equals(doc.doctype, null);
     t.done();