WebUI MD Refresh: Implement hover state for action buttons.

 - Add hover background color.
 - Add hover box shadow effect.

Bug: 832173
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I89eb5bb9c9b088ad85fbf5d22dd3735392d1ecd8
Reviewed-on: https://chromium-review.googlesource.com/1060554
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559278}

ui/webui/resources/cr_elements/paper_button_style_css.html

@@ -46,6 +46,15 @@
         border-color: rgb(210, 227, 252);
       }
 
+      paper-button.action-button:hover {
+        /* google-blue-600 with opacity. */
+        background-color: rgba(26,115,232, 0.9);
+        /* google-blue-500 with opacity */
+        box-shadow:
+          0 1px 1px 0 rgba(66, 133, 244, 0.54),
+          0 1px 3px 1px rgba(66, 133, 244, 0.30);
+      }
+
       paper-button.action-button {
         --paper-ripple-opacity: 0.32;
         --paper-button-ink-color: white;