[Local NTP] block image-related context menu items for thumbnails; CSS fixes.

In https://chromiumcodereview.appspot.com/473583002/ we removed a
'shadow' element in thumbnail <iframe> to improve customization. A side
effect is that this exposes the thumbnail image, thereby alow the user
to futilely access the chrome-search://thumb/#/# image below. This CL
adds back a transparent "blocker" in front of the thumbnail to restore
old behavior. Also making 2 CSS changes to:
- Fix fake box text positioning for RTL.
- Move top-margin back from #ntp-contents to #logo, for correctness.

BUG=411005

Review URL: https://codereview.chromium.org/542033002

Cr-Commit-Position: refs/heads/master@{#293650}

chrome/browser/resources/local_ntp/local_ntp.css

@@ -15,16 +15,7 @@ body {
   text-align: -webkit-center;
 }
 
-#ntp-contents.classical {
-  margin-top: 157px;
-}
-
-#ntp-contents.md {
-  margin-top: 157px;
-}
-
 .non-google-page #ntp-contents {
-  margin-top: 0;
   position: absolute;
   top: calc(50% - 155px);
   width: 100%;
@@ -50,6 +41,7 @@ body.fakebox-disable #fakebox > input {
   background-size: 269px 95px;
   height: 95px;
   margin-bottom: 24px;
+  margin-top: 157px;
   width: 269px;
 }
 
@@ -119,7 +111,7 @@ body[dir=rtl] #fakebox > input {
   margin-top: 1px;
   overflow: hidden;
   position: absolute;
-  text-align: left;
+  text-align: initial;
   text-overflow: ellipsis;
   vertical-align: middle;
   visibility: inherit;

chrome/browser/resources/local_ntp/most_visited_thumbnail.css

@@ -9,6 +9,7 @@ body {
 
 a {
   height: 100%;
+  position: relative;
   width: 100%;
 }
 
@@ -27,6 +28,13 @@ div {
   width: 90%;
 }
 
+span.blocker {
+  display: inline-block;
+  height: 100%;
+  position: absolute;
+  width: 100%;
+}
+
 img {
   height: 100%;
   width: 100%;

chrome/browser/resources/local_ntp/most_visited_thumbnail.js

@@ -45,6 +45,10 @@ window.addEventListener('DOMContentLoaded', function() {
         var link = createMostVisitedLink(
             params, data.url, data.title, undefined, data.direction,
             data.provider);
+        // Use blocker to prevent context menu from showing image-related items.
+        var blocker = document.createElement('span');
+        blocker.className = 'blocker';
+        link.appendChild(blocker);
         link.appendChild(image);
         displayLink(link);
       };