Enables AccessibleObjectFromEvent to retrieve the system caret

Each AXPlatformNodeWin, including the accessibility system caret, is assigned a unique ID on construction. Mistakenly, I was assigning a new ID instead of using the existing one when constructing the AXSystemCaretWin object which meant that all events were sent having the wrong object ID.
R=dmazzoni@chromium.org, aleventhal@chromium.org

Bug: 789176
Change-Id: I15681365ad0e461cf29789df9a4f69301e3e6104
Reviewed-on: https://chromium-review.googlesource.com/803858Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521109}

ui/accessibility/platform/ax_system_caret_win.cc

@@ -18,7 +18,10 @@ namespace ui {
 AXSystemCaretWin::AXSystemCaretWin(gfx::AcceleratedWidget event_target)
     : event_target_(event_target) {
   caret_ = static_cast<AXPlatformNodeWin*>(AXPlatformNodeWin::Create(this));
-  data_.id = GetNextAXPlatformNodeUniqueId();
+  // The caret object is not part of the accessibility tree and so doesn't need
+  // a node ID. A globally unique ID is used when firing Win events, retrieved
+  // via |unique_id|.
+  data_.id = -1;
   data_.role = AX_ROLE_CARET;
   // |get_accState| should return 0 which means that the caret is visible.
   data_.state = 0;
@@ -28,7 +31,7 @@ AXSystemCaretWin::AXSystemCaretWin(gfx::AcceleratedWidget event_target)
 
   if (event_target_) {
     ::NotifyWinEvent(EVENT_OBJECT_CREATE, event_target_, OBJID_CARET,
-                     -data_.id);
+                     -caret_->unique_id());
   }
 }
 
@@ -38,7 +41,7 @@ AXSystemCaretWin::~AXSystemCaretWin() {
   // retrieve the destroyed object in this stack frame.
   if (event_target_) {
     ::NotifyWinEvent(EVENT_OBJECT_DESTROY, event_target_, OBJID_CARET,
-                     -data_.id);
+                     -caret_->unique_id());
   }
 }
 
@@ -57,7 +60,7 @@ void AXSystemCaretWin::MoveCaretTo(const gfx::Rect& bounds) {
   data_.location = gfx::RectF(bounds);
   if (event_target_) {
     ::NotifyWinEvent(EVENT_OBJECT_LOCATIONCHANGE, event_target_, OBJID_CARET,
-                     -data_.id);
+                     -caret_->unique_id());
   }
 }