CSS4: Implement :placeholder-shown pseudo class

Source/core/css/SelectorChecker.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 748 matching lines @@
         if (ContainerNode* parent = element.parentElementOrDocumentFragment()) {
             if (m_mode == ResolvingStyle) {
                 parent->setChildrenAffectedByForwardPositionalRules();
                 parent->setChildrenAffectedByBackwardPositionalRules();
             }
             if (!parent->isFinishedParsingChildren())
                 return false;
             return isFirstOfType(element, element.tagQName()) && isLastOfType(element, element.tagQName());
         }
         break;
+    case CSSSelector::PseudoPlaceholderShown:
+        if (isHTMLTextFormControlElement(element)) {
+            if (m_mode == ResolvingStyle) {
+                if (ComputedStyle* style = context.elementStyle ? context.elementStyle : element.mutableComputedStyle())
+                    style->setUnique();

[esprehn, 2015/08/21 10:18:10]

This should not make you unique, change SharedStyleFinder to check
isPlaceholderVisible() when comparing input elements. Please don't copy pasta
webkit patches.

[ramya.v, 2015/08/25 10:24:45]

Done.

+            }
+            return toHTMLTextFormControlElement(element).isPlaceholderVisible();
+        }
+        break;
     case CSSSelector::PseudoNthChild:
         if (ContainerNode* parent = element.parentElementOrDocumentFragment()) {
             if (m_mode == ResolvingStyle)
                 parent->setChildrenAffectedByForwardPositionalRules();
             return selector.matchNth(nthChildIndex(element));
         }
         break;
     case CSSSelector::PseudoNthOfType:
         if (ContainerNode* parent = element.parentElementOrDocumentFragment()) {
             if (m_mode == ResolvingStyle)
@@ skipping 398 matching lines @@
 }
 
 bool SelectorChecker::matchesFocusPseudoClass(const Element& element)
 {
     if (InspectorInstrumentation::forcePseudoState(const_cast<Element*>(&element), CSSSelector::PseudoFocus))
         return true;
     return element.focused() && isFrameFocused(element);
 }
 
 }