[CR elements] migrate some <content> use to <slot>

This CL is a step toward using Polymer 2. It converts some uses of <content>
(the Polymer 1 way) to <slot> (the Polymer 2 way of slotting in items).

Bug: 739532
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I745ccd5ace1bde2254ce4daa63e254184281affe
Reviewed-on: https://chromium-review.googlesource.com/820556Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523920}

chrome/browser/resources/md_downloads/toolbar.html

@@ -39,7 +39,7 @@
     <cr-toolbar id="toolbar" page-name="$i18n{title}"
         search-prompt="$i18n{search}" clear-label="$i18n{clearSearch}"
         spinner-active="{{spinnerActive}}" on-search-changed="onSearchChanged_">
-      <button is="paper-icon-button-light" class="more-actions" id="moreActions"
+      <button is="paper-icon-button-light" id="moreActions"
           title="$i18n{moreActions}" class="dropdown-trigger"
           on-tap="onMoreActionsTap_">
         <iron-icon icon="cr:more-vert"></iron-icon>

ui/webui/resources/cr_elements/cr_profile_avatar_selector/cr_profile_avatar_selector_grid.html

@@ -12,7 +12,7 @@
         margin: calc(var(--avatar-spacing) / -2);
       }
     </style>
-    <content select=".avatar"></content>
+    <slot></slot>
   </template>
   <script src="cr_profile_avatar_selector_grid.js"></script>
 </dom-module>

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

@@ -160,7 +160,7 @@
     </div>
 
     <div id="rightContent">
-      <content select=".more-actions"></content>
+      <slot></slot>
     </div>
   </template>
   <script src="cr_toolbar.js"></script>