inspect.js: Disable selection on background div

Clicking on the inspect/close button too quickly in succession or right-
clicking the button would cause the whole div containing both buttons
to be highlighted which was annoying.

Before: https://imgur.com/a/OgdmeE7
After: https://imgur.com/a/nfjOrb4

Bug: chromium:1036789
Change-Id: I51328de5d1fa2ab232913770cc1cf594b019d660
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102581Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Auto-Submit: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750528}

chrome/browser/resources/inspect/inspect.css

@@ -257,6 +257,10 @@ img {
   margin-top: 5px;
 }
 
+.actions {
+  user-select: none;
+}
+
 .action {
   color: rgb(17, 85, 204);
   cursor: pointer;