Implement ::placeholder CSS selector.

third_party/WebKit/Source/core/css/CSSSelector.cpp

Index: third_party/WebKit/Source/core/css/CSSSelector.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSSelector.cpp b/third_party/WebKit/Source/core/css/CSSSelector.cpp
index 89d130750e122e0511d45c2ec8489e672408c01d..d8999b4951a500270aeb94715483829c836b62b6 100644
--- a/third_party/WebKit/Source/core/css/CSSSelector.cpp
+++ b/third_party/WebKit/Source/core/css/CSSSelector.cpp
@@ -357,6 +357,7 @@ const static NameToPseudoStruct pseudoTypeWithoutArgumentsMap[] = {
     {"optional", CSSSelector::PseudoOptional},
     {"out-of-range", CSSSelector::PseudoOutOfRange},
     {"past", CSSSelector::PseudoPastCue},
+    {"placeholder", CSSSelector::PseudoWebKitCustomElement},

[rune, 2016/11/18 09:53:23]

Making this a PseudoWebKitCustomElement means this pseudo element will get the
same treatment as -webkit-prefixed pseudo elements in certain parts of the code
like isPseudoClassValidAfterPseudoElement(). For the mentioned method it means
we're allowing user action pseudo classes after the element. What would the code
look like if we actually assigned a separate pseudo element enum for this?

[tkent, 2016/11/21 05:36:02]

Firefox and Safari don't accept user-action pseudo classes after ::placeholder. 
So this approach wasn't good.

Patch Set 2 introduces new separated enum.  Would you take another look at it
please?
I also added tests.

     {"placeholder-shown", CSSSelector::PseudoPlaceholderShown},
     {"read-only", CSSSelector::PseudoReadOnly},
     {"read-write", CSSSelector::PseudoReadWrite},
@@ -437,6 +438,12 @@ static CSSSelector::PseudoType nameToPseudoType(const AtomicString& name,
   return static_cast<CSSSelector::PseudoType>(match->type);
 }
 
+AtomicString CSSSelector::aliasForShadowPseudoId(const AtomicString& name) {

[rune, 2016/11/18 09:53:23]

Using the term alias here could be confusing since you're not implementing it as
an alias in terms of the whatwg compat spec, but just sharing code between the
implementations.

Not sure what a better name would be.

+  if (name == "-webkit-input-placeholder")
+    return "placeholder";
+  return nullAtom;
+}
+
 #ifndef NDEBUG
 void CSSSelector::show(int indent) const {
   printf("%*sselectorText(): %s\n", indent, "", selectorText().ascii().data());