Revert "Marks the document object as busy while it is still loading"

This reverts commit e19fa28d.

Reason for revert: Cause of https://crbug.com/903402

Original change's description:
> Marks the document object as busy while it is still loading
> 
> Screen readers want to have a reliable way to know whether a document is still loading so as to ignore events coming from it.
> The current change can be made irrespective to whether we will mark the document as loaded when HTML parsing has started or when the "interactive" readyState has been entered.
> 
> R=​dmazzoni@chromium.org, aleventhal@chromium.org
> 
> Bug: 897177
> Change-Id: I5d734ff9d32c53b7ba12a6d5d4eeb74b26ba0a02
> Reviewed-on: https://chromium-review.googlesource.com/c/1295769
> Commit-Queue: Nektarios Paisios <nektar@chromium.org>
> Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#606147}

TBR=dmazzoni@chromium.org,nektar@chromium.org,aleventhal@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 897177
Change-Id: Ib13f772ecf7fe300afbb48e16952f65bf0d0728a
Reviewed-on: https://chromium-review.googlesource.com/c/1327721Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606658}

content/browser/accessibility/browser_accessibility_manager.cc

@@ -207,7 +207,8 @@ void BrowserAccessibilityManager::FireFocusEventsIfNeeded() {
   // or for the document to actually finish loading.
   if (focus && focus == focus->manager()->GetRoot() &&
       focus->PlatformChildCount() == 0 &&
-      focus->GetBoolAttribute(ax::mojom::BoolAttribute::kBusy)) {
+      !focus->GetBoolAttribute(ax::mojom::BoolAttribute::kBusy) &&
+      !focus->manager()->GetTreeData().loaded) {
     focus = nullptr;
   }
 

content/renderer/accessibility/blink_ax_tree_source.cc

@@ -443,9 +443,6 @@ void BlinkAXTreeSource::SerializeNode(WebAXObject src,
   TRACE_EVENT1("accessibility", "BlinkAXTreeSource::SerializeNode", "role",
                ui::ToString(dst->role));
 
-  if (src.Equals(root()) && !root().IsLoaded())
-    dst->AddBoolAttribute(ax::mojom::BoolAttribute::kBusy, true);
-
   WebAXObject offset_container;
   WebFloatRect bounds_in_container;
   SkMatrix44 container_transform;