AXSelection: Added the ability for a web app to cancel an accessibility SetSelection action

This functionality is based on the Selection API draft specification and is already supported in Blink by other types of user initiated selection actions:
https://w3c.github.io/selection-api/#selectstart-event

Tests are in the following patch:
https://chromium-review.googlesource.com/c/chromium/src/+/1350195/

R=dmazzoni@chromium.org

Bug: 639340
Change-Id: I5a7793c71b02f2e92fafb7fb9731b6ebf288a412
Tested: with unit tests in another patch
Reviewed-on: https://chromium-review.googlesource.com/c/1347214
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611400}

third_party/blink/renderer/modules/accessibility/ax_selection.cc

@@ -4,8 +4,11 @@
 
 #include "third_party/blink/renderer/modules/accessibility/ax_selection.h"
 
+#include "third_party/blink/renderer/core/dom/events/event.h"
+#include "third_party/blink/renderer/core/dom/node.h"
 #include "third_party/blink/renderer/core/dom/range.h"
 #include "third_party/blink/renderer/core/editing/frame_selection.h"
+#include "third_party/blink/renderer/core/editing/position.h"
 #include "third_party/blink/renderer/core/editing/position_with_affinity.h"
 #include "third_party/blink/renderer/core/editing/selection_template.h"
 #include "third_party/blink/renderer/core/editing/set_selection_options.h"
@@ -15,6 +18,18 @@
 
 namespace blink {
 
+namespace {
+
+DispatchEventResult DispatchSelectStart(Node* node) {
+  if (!node)
+    return DispatchEventResult::kNotCanceled;
+
+  return node->DispatchEvent(
+      *Event::CreateCancelableBubble(event_type_names::kSelectstart));
+}
+
+}  // namespace
+
 //
 // AXSelection::Builder
 //
@@ -80,7 +95,7 @@ const AXSelection AXSelection::Builder::Build() {
 //
 
 // static
-void AXSelection::ClearCurrentSelection(const Document& document) {
+void AXSelection::ClearCurrentSelection(Document& document) {
   LocalFrame* frame = document.GetFrame();
   if (!frame)
     return;
@@ -223,10 +238,10 @@ const SelectionInDOMTree AXSelection::AsSelection(
   return selection_builder.Build();
 }
 
-void AXSelection::Select(const AXSelectionBehavior selection_behavior) {
+bool AXSelection::Select(const AXSelectionBehavior selection_behavior) {
   if (!IsValid()) {
     NOTREACHED() << "Trying to select an invalid accessibility selection.";
-    return;
+    return false;
   }
 
   const SelectionInDOMTree selection = AsSelection(selection_behavior);
@@ -234,23 +249,31 @@ void AXSelection::Select(const AXSelectionBehavior selection_behavior) {
   Document* document = selection.Base().GetDocument();
   if (!document) {
     NOTREACHED();
-    return;
+    return false;
   }
 
   LocalFrame* frame = document->GetFrame();
   if (!frame) {
     NOTREACHED();
-    return;
+    return false;
   }
 
   FrameSelection& frame_selection = frame->Selection();
   if (!frame_selection.IsAvailable())
-    return;
+    return false;
+
+  // See the following section in the Selection API Specification:
+  // https://w3c.github.io/selection-api/#selectstart-event
+  if (DispatchSelectStart(selection.Extent().ComputeContainerNode()) !=
+      DispatchEventResult::kNotCanceled) {
+    return false;
+  }
 
   SetSelectionOptions::Builder options_builder;
   options_builder.SetIsDirectional(true)
       .SetShouldCloseTyping(true)
-      .SetShouldClearTypingStyle(true);
+      .SetShouldClearTypingStyle(true)
+      .SetSetSelectionBy(SetSelectionBy::kUser);
   frame_selection.ClearDocumentCachedRange();
   frame_selection.SetSelection(selection, options_builder.Build());
 
@@ -281,6 +304,7 @@ void AXSelection::Select(const AXSelectionBehavior selection_behavior) {
                   selection.Base().ComputeOffsetInContainerNode());
   }
   frame_selection.CacheRangeOfDocument(range);
+  return true;
 }
 
 String AXSelection::ToString() const {

third_party/blink/renderer/modules/accessibility/ax_selection.h

@@ -39,7 +39,7 @@ class MODULES_EXPORT AXSelection final {
  public:
   class Builder;
 
-  static void ClearCurrentSelection(const Document&);
+  static void ClearCurrentSelection(Document&);
 
   static AXSelection FromCurrentSelection(
       const Document&,
@@ -66,8 +66,10 @@ class MODULES_EXPORT AXSelection final {
       const AXSelectionBehavior =
           AXSelectionBehavior::kExtendToValidDOMRange) const;
 
-  // Tries to set the DOM selection to this.
-  void Select(
+  // Tries to set the DOM selection to this. Returns |false| if the selection
+  // has been cancelled via the "selectionstart" event or if the selection could
+  // not be set for any other reason.
+  bool Select(
       const AXSelectionBehavior = AXSelectionBehavior::kExtendToValidDOMRange);
 
   // Returns a string representation of this object.