WebUI Polymer 2: Use !important in cr-toast to make ::slotted style t…

…ake effect.

Specificity of ::slotted() rules is different in Shadow DOM v0 and v1. The latter
is the correct specced behavior, but the code so far relied on the former.

Bug: 896674
Change-Id: I0eb2fc0925e1cb83db732e89d958acab4a8fc5fb
Reviewed-on: https://chromium-review.googlesource.com/c/1290198
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601706}

ui/webui/resources/cr_elements/cr_toast/cr_toast.html

@@ -34,16 +34,16 @@
       }
 
       :host ::slotted(paper-button) {
-        background-color: transparent;
-        border: none;
-        color: var(--google-blue-300);
-        margin-inline-start: 32px;
-        min-width: 52px;
-        padding: 8px;
+        background-color: transparent !important;
+        border: none !important;
+        color: var(--google-blue-300) !important;
+        margin-inline-start: 32px !important;
+        min-width: 52px !important;
+        padding: 8px !important;
       }
 
       :host ::slotted(paper-button:hover) {
-        background-color: transparent;
+        background-color: transparent !important;
       }
     </style>
     <slot></slot>