Make about:blank page have a charset of UTF-8

This brings us into alignment with the Fetch Standard as well as Firefox and
Edge.

Bug: 165083, 874536
Change-Id: Id06d07d301aeae8bd0c48bfe94df68cf1ed68db9
Reviewed-on: https://chromium-review.googlesource.com/1176263Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Timothy Gu <timothygu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584357}

third_party/WebKit/LayoutTests/external/wpt/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/creating_browsing_context_test_01.html

@@ -20,6 +20,7 @@ test(function() {
   var win = window.open('', '', 'height=1,width=1');
   this.add_cleanup(function() { win.close(); });
   assert_equals(win.location.href, 'about:blank', 'win.location.href');
+  assert_equals(win.document.charset, 'UTF-8', 'win.document.charset');
 }, 'first argument: empty url');
 
 test(function () {

third_party/WebKit/LayoutTests/external/wpt/html/browsers/windows/browsing-context-expected.txt

-This is a testharness.js-based test.
-FAIL Check that browsing context has new, ready HTML document assert_equals: The document's encoding should be 'UTF-8'. expected "UTF-8" but got "windows-1252"
-PASS Check that new document nodes extant, empty
-PASS Check the document properties corresponding to the creator browsing context
-Harness: the test ran to completion.
-

third_party/WebKit/LayoutTests/inspector-protocol/dom-snapshot/dom-snapshot-getSnapshot-origin-url-expected.txt

@@ -173,7 +173,7 @@ Tests that DOMSnapshot.getSnapshot records origin url of dom nodes generated by
       "layoutNodeIndex": 7,
       "documentURL": "<string>",
       "baseURL": "<string>",
-      "documentEncoding": "windows-1252",
+      "documentEncoding": "UTF-8",
       "frameId": "<string>"
     },
     {

third_party/blink/renderer/core/loader/document_loader.cc

@@ -888,6 +888,7 @@ bool DocumentLoader::MaybeLoadEmpty() {
       !GetFrameLoader().StateMachine()->CreatingInitialEmptyDocument())
     request_.SetURL(BlankURL());
   response_ = ResourceResponse(request_.Url(), "text/html");
+  response_.SetTextEncodingName("utf-8");
   FinishedLoading(CurrentTimeTicks());
   return true;
 }