[uber page] Convert 'Under the Hood' page to a sub-page.

chrome/browser/resources/options2/options_page.css

 #main-content {
   display: -webkit-box;
   position: absolute;
   left: 0;
   right: 0;
   top: 0;
   bottom: 0;
 }
 
 #mainview {
@@ skipping 253 matching lines @@
 #subpage-sheet-container-1 {
   -webkit-padding-start: 40px;
   z-index: 5;
 }
 
 #subpage-sheet-container-2 {
   -webkit-padding-start: 80px;
   z-index: 10;
 }
 
+#subpage-sheet-container-3 {
+  -webkit-padding-start: 120px;
+  z-index: 15;
+}
+
 .subpage-sheet {
   -webkit-box-shadow: #666 0px 2px 5px;
   background-color: white;
   border-left: 1px solid #b8b8b8;
   box-sizing: border-box;
   min-height: 100%;
   width: 100%;
   min-width: 651px;
 }
 
@@ skipping 505 matching lines @@
       url('chrome://theme/IDR_CONTROLLED_SETTING_RECOMMENDED_LARGE');
 }
 
 html[dir='rtl'] .controlled-setting-bubble-text {
   background-position: right top;
 }
 
 .controlled-setting-bubble-action {
   padding: 0 !important;
 }