Handles aria-dropeffect after getting its value from blink.

content/browser/accessibility/browser_accessibility_cocoa.mm

Index: content/browser/accessibility/browser_accessibility_cocoa.mm
diff --git a/content/browser/accessibility/browser_accessibility_cocoa.mm b/content/browser/accessibility/browser_accessibility_cocoa.mm
index b09e4d6b5d81bef64cac70da14030ec94443ba73..17626c9adb888a32654b274eea33481548a070f7 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.mm
+++ b/content/browser/accessibility/browser_accessibility_cocoa.mm
@@ -109,6 +109,7 @@ NSDictionary* attributeToMethodNameMap = nil;
     { @"AXARIABusy", @"ariaBusy" },
     { @"AXARIALive", @"ariaLive" },
     { @"AXARIARelevant", @"ariaRelevant" },
+    { @"AXDropEffects", @"dropeffect" },
     { @"AXGrabbed", @"grabbed" },
     { @"AXInvalid", @"invalid" },
     { @"AXLoaded", @"loaded" },
@@ -327,6 +328,11 @@ NSDictionary* attributeToMethodNameMap = nil;
   return nil;
 }
 
+- (NSString*)dropeffect {
+  return NSStringForStringAttribute(
+      browserAccessibility_, ui::AX_ATTR_DROPEFFECT);
+}
+
 - (NSNumber*)enabled {
   return [NSNumber numberWithBool:
       GetState(browserAccessibility_, ui::AX_STATE_ENABLED)];
@@ -1392,6 +1398,14 @@ NSDictionary* attributeToMethodNameMap = nil;
         @"AXARIABusy",
         nil]];
   }
+
+  if (browserAccessibility_->HasStringAttribute(
+          ui::AX_ATTR_DROPEFFECT)) {
+    [ret addObjectsFromArray:[NSArray arrayWithObjects:
+        @"AXDropEffects",
+        nil]];
+  }
+
   // Add aria-grabbed attribute only if it has true.
   if (browserAccessibility_->HasBoolAttribute(ui::AX_ATTR_GRABBED)) {
     [ret addObjectsFromArray:[NSArray arrayWithObjects: