WebUI MD Refresh: Fix disabled button text color regression.

This was caused because of a previous CL increased specificity of the
paper-button.action-button to paper-button:not([raised]).action-button
and similar increase should happen for the disabled case.

Bug: 845791
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I1bd432194165857cfa91dc456cbe4c1eb11e6871
Reviewed-on: https://chromium-review.googlesource.com/1070498Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561271}

ui/webui/resources/cr_elements/paper_button_style_css.html

@@ -77,7 +77,7 @@
         color: white;
       }
 
-      paper-button[disabled] {
+      paper-button:not([raised])[disabled] {
         background-color: white;
         border-color: var(--disabled-color);
         color: rgb(128, 134, 139);