[Devtools] Fix the before content for AXBreadcrumbs not to speak " ".

Bug: 560525
Change-Id: I9096609606dff3230ccaebfc763dc5ec43dc761a
Reviewed-on: https://chromium-review.googlesource.com/764579
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#525243}

third_party/WebKit/Source/devtools/front_end/accessibility/axBreadcrumbs.css

@@ -45,13 +45,13 @@
     -webkit-mask-size: 30px 10px;
     -webkit-mask-repeat: no-repeat;
     background-color: rgb(48, 57, 66);
-    content: " ";
-    color: transparent;
+    content: "";
     text-shadow: none;
     margin-right: -2px;
     height: 12px;
     width: 14px;
     position: absolute;
+    display: inline-block;
 }
 
 @media (-webkit-min-device-pixel-ratio: 1.1) {