DevTools: fix color swatch copy icon color

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

Bug: 959001
Change-Id: Idc6e7303821e4180ec90831341a29aebe594fcc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597283Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Erik Luo <luoe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659295}

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

@@ -282,7 +282,7 @@
     background-color: rgba(0, 0, 0, .5);
 }
 
-.copy-color-icon {
+[is=ui-icon].icon-mask.copy-color-icon {
     background-color: white;
 }