Fire accessible event on web area object when scrolling

Scroll events for the main document were accidentally removed in bug
371039 - this is a better way to restore them by firing them on the
web area, rather than the ScrollView/FrameView.

This brings back the "scroll" sounds on Android, and also makes Android
update the position of accessibility focus after scrolling.

BUG=371039, 374316, 414291

Review URL: https://codereview.chromium.org/662913002

git-svn-id: svn://svn.chromium.org/blink/trunk@184002 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/accessibility/scroll-window-horiz-sends-notification-expected.txt

@@ -7,7 +7,7 @@ On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE
 
 
 PASS window.pageXOffset is 0
-Got notification on scroll area
+Got notification on web area
 PASS window.pageXOffset is 500
 PASS successfullyParsed is true
 

third_party/WebKit/LayoutTests/accessibility/scroll-window-horiz-sends-notification.html

@@ -29,8 +29,8 @@ if (window.testRunner && window.accessibilityController) {
     shouldBe("window.pageXOffset", "0");
 
     accessibilityController.addNotificationListener(function (target, notification) {
-        if (target.role == 'AXRole: AXScrollArea') {
-            debug('Got notification on scroll area');
+        if (target.role == 'AXRole: AXWebArea' && notification == 'ScrollPositionChanged') {
+            debug('Got notification on web area');
             accessibilityController.removeNotificationListener();
             shouldBe("window.pageXOffset", "500");
             finishJSTest();

third_party/WebKit/LayoutTests/accessibility/scroll-window-sends-notification-expected.txt

@@ -7,7 +7,7 @@ On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE
 
 
 PASS window.pageYOffset is 0
-Got notification on scroll area
+Got notification on web area
 PASS window.pageYOffset is 500
 PASS successfullyParsed is true
 

third_party/WebKit/LayoutTests/accessibility/scroll-window-sends-notification.html

@@ -29,8 +29,8 @@ if (window.testRunner && window.accessibilityController) {
     shouldBe("window.pageYOffset", "0");
 
     accessibilityController.addNotificationListener(function (target, notification) {
-        if (target.role == 'AXRole: AXScrollArea') {
-            debug('Got notification on scroll area');
+        if (target.role == 'AXRole: AXWebArea' && notification == 'ScrollPositionChanged') {
+            debug('Got notification on web area');
             accessibilityController.removeNotificationListener();
             shouldBe("window.pageYOffset", "500");
             finishJSTest();

third_party/WebKit/Source/core/accessibility/AXObjectCache.cpp

@@ -1036,9 +1036,13 @@ void AXObjectCache::handleScrolledToAnchor(const Node* anchorNode)
     postPlatformNotification(AXObject::firstAccessibleObjectFromNode(anchorNode), AXScrolledToAnchor);
 }
 
-void AXObjectCache::handleScrollPositionChanged(FrameView* scrollView)
+void AXObjectCache::handleScrollPositionChanged(FrameView* frameView)
 {
-    postPlatformNotification(getOrCreate(scrollView), AXScrollPositionChanged);
+    // Prefer to fire the scroll position changed event on the frame view's child web area, if possible.
+    AXObject* targetAXObject = getOrCreate(frameView);
+    if (targetAXObject && !targetAXObject->children().isEmpty())
+        targetAXObject = targetAXObject->children()[0].get();
+    postPlatformNotification(targetAXObject, AXScrollPositionChanged);
 }
 
 void AXObjectCache::handleScrollPositionChanged(RenderObject* renderObject)