Implement caching asynchronous accessibility hit testing.

content/browser/accessibility/browser_accessibility_manager.h

Index: content/browser/accessibility/browser_accessibility_manager.h
diff --git a/content/browser/accessibility/browser_accessibility_manager.h b/content/browser/accessibility/browser_accessibility_manager.h
index 91b31c75814d78f63c3bebe443f8b16a34049348..f6b98189e2e779e57a3fdf711448fa6ae6f80a64 100644
--- a/content/browser/accessibility/browser_accessibility_manager.h
+++ b/content/browser/accessibility/browser_accessibility_manager.h
@@ -383,6 +383,16 @@ class CONTENT_EXPORT BrowserAccessibilityManager : public ui::AXTreeDelegate {
   // Get a snapshot of the current tree as an AXTreeUpdate.
   ui::AXTreeUpdate SnapshotAXTreeForTesting();
 
+  // Given a point in screen coordinates, trigger an asynchronous hit test
+  // but return the best possible match instantly.
+  //
+  //
+  BrowserAccessibility* CachingAsyncHitTest(const gfx::Point& screen_point);
+
+  // Called in response to a hover event, caches the result for the next
+  // call to CachingAsyncHitTest().
+  void CacheHitTestResult(BrowserAccessibility* hit_test_result);
+
  protected:
   BrowserAccessibilityManager(
       BrowserAccessibilityDelegate* delegate,
@@ -453,6 +463,14 @@ class CONTENT_EXPORT BrowserAccessibilityManager : public ui::AXTreeDelegate {
   BrowserAccessibility* last_focused_node_;
   BrowserAccessibilityManager* last_focused_manager_;
 
+  // These cache the AX tree ID, node ID, and global screen bounds of the
+  // last object found by an asynchronous hit test. Subsequent hit test
+  // requests that remain within this object's bounds will return the same
+  // object, but will also trigger a new asynchronous hit test request.
+  int last_hover_ax_tree_id_;
+  int last_hover_node_id_;
+  gfx::Rect last_hover_bounds_;
+
   // True if the root's parent is in another accessibility tree and that
   // parent's child is the root. Ensures that the parent node is notified
   // once when this subtree is first connected.