WebUI Dark Mode: increase opacity for toolbar search placeholder text

Screenshots: https://imgur.com/a/7sab9SR

R=dpapad@chromium.org
BUG=950814

Change-Id: I48ce37a3b7fd369fea9f6d3852017a0c423a32f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559398
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649281}

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_search_field.html

@@ -37,9 +37,14 @@
       }
 
       #prompt {
+        --cr-toolbar-search-field-prompt-opacity: .7;
         opacity: 0;
       }
 
+      :host-context([dark]) #prompt {
+        --cr-toolbar-search-field-prompt-opacity: 1;
+      }
+
       paper-spinner-lite {
         @apply --cr-icon-height-width;
         --paper-spinner-color: white;
@@ -111,7 +116,7 @@
 
       :host(:not([narrow]):not([showing-search])) #icon,
       :host(:not([narrow])) #prompt {
-        opacity: 0.7;
+        opacity: var(--cr-toolbar-search-field-prompt-opacity);
       }
 
       :host([narrow]:not([showing-search])) #searchTerm {