WebUI MD Refresh: Fix focused state on Extensions page.

It turns out that CSS mixins inheritance and precedence is more fragile than normal
CSS classes. The order of style includes affects the precedence, which caused the
styling to look correct on Settings, but incorrect on Extensions
(happens only with optimize_webui=true).

Bug: 837140
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I22b592315964c3101d25c68dd5f5f1c29d75b347
Reviewed-on: https://chromium-review.googlesource.com/1035785Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554943}

ui/webui/resources/cr_elements/paper_button_style_css.html

@@ -15,13 +15,6 @@
         /* Blue-ish color used either as a background or as a text color,
            depending on the type of button. */
         --text-or-bg-color: rgb(26, 115, 232);
-
-        --paper-button-flat-keyboard-focus: {
-          background: initial;
-          border-color: var(--flat-keyboard-focus-border-color);
-          font-weight: 500;
-        };
-
         --paper-button-ink-color: var(--paper-grey-600);
 
         border: 1px solid rgb(218, 220, 224);
@@ -36,6 +29,13 @@
         text-transform: none;
       }
 
+      paper-button.keyboard-focus,
+      paper-button.action-button.keyboard-focus {
+        border-color: var(--flat-keyboard-focus-border-color);
+        /* Override default paper-button's internal font-weight style. */
+        font-weight: 500;
+      }
+
       paper-button:not(.action-button):hover {
         background-color: rgba(66,133,244, 0.04);
         border-color: rgb(210, 227, 252);
@@ -47,11 +47,6 @@
         background: var(--text-or-bg-color) padding-box;
         border-color: transparent;
         color: white;
-        --paper-button-flat-keyboard-focus: {
-          background: var(--text-or-bg-color) padding-box;
-          border-color: var(--flat-keyboard-focus-border-color);
-          font-weight: 500;
-        };
       }
 
       paper-button[disabled] {