DevTools: fix cursor style and disabled state for TextPrompt

- Hovering cursor over placeholder text now has correct 'I-beam' style.
- Disabled TextPrompts now look visually different, more transparent,
  consistent with the disabled style on disabled ToolbarButtons.

Screenshot: https://imgur.com/a/FgN8w

Bug: 773964
Change-Id: I87de99d01ac95653dba3eb62299a27ea2ff19de4
Reviewed-on: https://chromium-review.googlesource.com/744725Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512700}

third_party/WebKit/Source/devtools/front_end/ui/TextPrompt.js

@@ -210,6 +210,7 @@ UI.TextPrompt = class extends Common.Object {
       this._element.setAttribute('contenteditable', 'plaintext-only');
     else
       this._element.removeAttribute('contenteditable');
+    this._element.classList.toggle('disabled', !enabled);
   }
 
   _removeFromElement() {

third_party/WebKit/Source/devtools/front_end/ui/textPrompt.css

@@ -46,6 +46,15 @@
     display: block;
 }
 
+::content .text-prompt {
+    cursor: text;
+}
+
+::content .text-prompt.disabled {
+    opacity: 0.5;
+    cursor: default;
+}
+
 .text-prompt-editing ::content br {
     display: none;
 }