Implement ::placeholder CSS selector.

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

+<style>
+textarea {
+    color: darkGray;
+}
+
+.red {
+    color: rgb(100, 0, 0);
+}
+
+.blue {
+    color: blue;
+    font-weight: bold;
+    font-size: 16px;
+    width: 135px;
+    height: 30px;
+}
+
+.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:
+<textarea>default</textarea>
+<textarea disabled>default disabled</textarea>
+</div>
+
+<div>
+::-webkit-input-placeholder:
+<textarea class="red">text</textarea>
+<textarea class="red" disabled>disabled text</textarea>
+</div>
+
+<div>
+::placeholder:
+<textarea class="blue">text</textarea>
+<textarea class="blue" disabled>disabled text</textarea>
+</div>
+
+<div>
+Both:
+<textarea class="prefixed-unprefixed">unprefixed</textarea>
+<textarea class="unprefixed-prefixed">prefixed</textarea>
+</div>