DevTools: Fix broken css in spectrum.css

There was a missing }

Change-Id: I609a0e16968091f5fc5d487ad23059204299bd8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617204Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661035}

third_party/blink/renderer/devtools/front_end/color_picker/spectrum.css

@@ -622,3 +622,4 @@ button.background-color-picker.active [is=ui-icon].largeicon-eyedropper.icon-mas
 
 button.background-color-picker:hover [is=ui-icon].largeicon-eyedropper.icon-mask {
     background-color: #333;
+}