[MD settings] google-grey to paper-grey

This CL replaces some uses of the non-existent
--google-grey-600 with --paper-grey-600

BUG=589630
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2010283003
Cr-Commit-Position: refs/heads/master@{#398150}

chrome/browser/resources/settings/passwords_and_forms_page/passwords_shared_css.html

@@ -23,7 +23,7 @@
         -webkit-margin-start: 20px;
         -webkit-padding-end: 0;
         -webkit-padding-start: 0;
-        color: var(--google-grey-600);
+        color: var(--paper-grey-600);
         width: 20px;
       }
 

chrome/browser/resources/settings/settings_page/settings_section.html

@@ -16,7 +16,7 @@
       }
 
       #header .title {
-        color: var(--paper-grey-600);
+        color: rgb(104, 113, 116);  /* Custom color from bettes@ */
         font-size: 13px;
         font-weight: 500;
       }