Implement ::placeholder CSS selector.

third_party/WebKit/LayoutTests/fast/forms/text/placeholder-pseudo-style-expected.html

+<style>
+input {
+    color: darkGray;
+}
+
+input.red {
+    color: rgb(100, 0, 0);
+}
+
+.blue {
+    color: blue;
+}
+
+.prefixed-unprefixed {
+    color: red;
+}
+.prefixed-unprefixed {
+    color: blue;
+}
+
+.unprefixed-prefixed {
+    color: blue;
+}
+.unprefixed-prefixed {
+    color: red;
+}
+</style>
+This tests that you can set the placeholder text color.
+
+<div>
+Default style:
+<input value="default">
+<input value="default disabled">
+</div>
+
+<div>
+::-webkit-input-placeholder:
+<input class="red" value="text">
+<input class="red" type="search" value="search">
+<input class="red" type="text" value="password">
+<input class="red" disabled value="disabled text">
+</div>
+
+<div>
+::placeholder:
+<input class="blue" value="text">
+<input class="blue" type="search" value="search">
+<input class="blue" type="text" value="password">
+<input class="blue" disabled value="disabled text">
+</div>
+
+<div>
+Both:
+<input class="prefixed-unprefixed" value="unprefixed">
+<input class="unprefixed-prefixed" value="prefixed">
+</div>