Expose html placeholder attribute as AXPlaceholder: <value> on MAC.

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 92515d542894d31b8b2baa16a18ff8c17eb9871f..a40761c2df72b09a7093c3f4e3c83e5235dbfb06 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.mm
+++ b/content/browser/accessibility/browser_accessibility_cocoa.mm
@@ -111,6 +111,7 @@ NSDictionary* attributeToMethodNameMap = nil;
     { @"AXInvalid", @"invalid" },
     { @"AXLoaded", @"loaded" },
     { @"AXLoadingProgress", @"loadingProgress" },
+    { @"AXPlaceholder", @"placeholder" },
     { @"AXRequired", @"required" },
     { @"AXVisited", @"visited" },
   };
@@ -401,6 +402,11 @@ NSDictionary* attributeToMethodNameMap = nil;
   return invalid;
 }
 
+- (NSString*)placeholder {
+  return NSStringForStringAttribute(
+      browserAccessibility_, ui::AX_ATTR_PLACEHOLDER);
+}
+
 - (void)addLinkedUIElementsFromAttribute:(ui::AXIntListAttribute)attribute
                                    addTo:(NSMutableArray*)outArray {
   const std::vector<int32>& attributeValues =
@@ -1358,6 +1364,11 @@ NSDictionary* attributeToMethodNameMap = nil;
         NSAccessibilityOrientationAttribute, nil]];
   }
 
+  if (browserAccessibility_->HasStringAttribute(ui::AX_ATTR_PLACEHOLDER)) {
+    [ret addObjectsFromArray:[NSArray arrayWithObjects:
+        @"AXPlaceholder", nil]];
+  }
+
   if (GetState(browserAccessibility_, ui::AX_STATE_REQUIRED)) {
     [ret addObjectsFromArray:[NSArray arrayWithObjects:
         @"AXRequired", nil]];