DevTools: Align TextPrompt with <input> scroll behavior

This lets you scroll the text prompt horizontally by selecting text
with the mouse.

Bug: 800038
Change-Id: I6e6970a6998530d3355c458d826923b471616e4a
Reviewed-on: https://chromium-review.googlesource.com/1072417Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561684}

third_party/blink/renderer/devtools/front_end/ui/textPrompt.css

@@ -46,6 +46,11 @@
 
 ::content .text-prompt {
     cursor: text;
+    overflow-x: visible;
+}
+
+::content .text-prompt::-webkit-scrollbar {
+    display: none;
 }
 
 ::content .text-prompt.disabled {