Android accessibility: improve handling of same-page links

This is a new attempt at fixing bug 657157, previous attempts were
reverted due to regressions.

When setting accessibility focus to a node, auto-focus it if it's a link.
This is needed for some sites that have skip links that are only visible
when focused.

In addition, when following a same-page link, move accessibility focus
to the new target location on the page only after a short delay. This
avoids a race condition due to focus also changing at the same time.

Tested manually on three pages mentioned in bug 657157:
1. getbootstrap.com, which has a same-page link that's only visible
   when focused
2. http://jsfiddle.net/mev0c4dt/show/ - a test page for a regression
   that happened with a previous attempt to fix this bug
3. https://codepen.io/artesea/pen/jVdLXP - a test page for a regression
   that happened with a previous attempt to fix this bug

Bug: 657157
Change-Id: I791544af3074f5aac5116c6695332d8178da21d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1873964Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710150}

content/browser/accessibility/web_contents_accessibility_android.cc

@@ -1099,6 +1099,11 @@ void WebContentsAccessibilityAndroid::MoveAccessibilityFocus(
   // as that will result in loading inline text boxes for the whole tree.
   if (node != node->manager()->GetRoot())
     node->manager()->LoadInlineTextBoxes(*node);
+
+  // Auto-focus links, because some websites have skip links that are
+  // only visible when focused. See http://crbug.com/657157
+  if (node->IsLink())
+    node->manager()->SetFocus(*node);
 }
 
 bool WebContentsAccessibilityAndroid::IsSlider(JNIEnv* env,

content/public/android/java/src/org/chromium/content/browser/accessibility/WebContentsAccessibilityImpl.java

@@ -66,6 +66,7 @@ public class WebContentsAccessibilityImpl extends AccessibilityNodeProvider
     private static final String ACTION_ARGUMENT_SET_TEXT_CHARSEQUENCE =
             "ACTION_ARGUMENT_SET_TEXT_CHARSEQUENCE";
     private static final int WINDOW_CONTENT_CHANGED_DELAY_MS = 500;
+    private static final int SCROLLED_TO_ANCHOR_DELAY_MS = 500;
 
     // Constants from AccessibilityNodeInfo defined in the M SDK.
     // Source: https://developer.android.com/reference/android/R.id.html
@@ -100,6 +101,7 @@ public class WebContentsAccessibilityImpl extends AccessibilityNodeProvider
     protected int mAccessibilityFocusId;
     protected int mSelectionNodeId;
     private Runnable mSendWindowContentChangedRunnable;
+    private Runnable mScrolledToAnchorRunnable;
     private View mAutofillPopupView;
     private CaptioningController mCaptioningController;
 
@@ -1048,8 +1050,27 @@ public class WebContentsAccessibilityImpl extends AccessibilityNodeProvider
     }
 
     @CalledByNative
-    private void handleScrolledToAnchor(int id) {
-        moveAccessibilityFocusToId(id);
+    private void handleScrolledToAnchor(final int id) {
+        // "Scrolled to anchor" means that the user followed a same-page link;
+        // the id here is of the element that was scrolled into view, and that
+        // should be where accessibility focus lands.
+        //
+        // However, in practice there's a race condition because following a
+        // same-page link often results in a focus change from the same-page link
+        // that was focused previously.
+        //
+        // As a result, it works better to move accessibility focus to the new
+        // location after a short delay, after the focus change.
+        if (mScrolledToAnchorRunnable != null) return;
+        mScrolledToAnchorRunnable = new Runnable() {
+            @Override
+            public void run() {
+                moveAccessibilityFocusToId(id);
+                mScrolledToAnchorRunnable = null;
+            }
+        };
+
+        mView.postDelayed(mScrolledToAnchorRunnable, SCROLLED_TO_ANCHOR_DELAY_MS);
     }
 
     @CalledByNative