[Local NTP] Fixing thumbnail aspect ratio, and store taller thumbnails.

Previously the local NTP thumbnails were stretched. We now make preserve
ratio preserved by setting height to "auto", but keep min-height to 100%
so if thumbnail is too short we won't see blank space.
To accommotate Material Design thumbnails, we make stored thumbnails
taller. In the thumbnail <iframe>, the extra height is hidden from view.

BUG=414878

Review URL: https://codereview.chromium.org/564973003

Cr-Commit-Position: refs/heads/master@{#295155}

chrome/browser/resources/local_ntp/most_visited_thumbnail.css

@@ -36,6 +36,7 @@ span.blocker {
 }
 
 img {
-  height: 100%;
+  height: auto;
+  min-height: 100%;
   width: 100%;
 }

chrome/browser/thumbnails/thumbnail_service_impl.cc

@@ -21,7 +21,7 @@ namespace {
 
 // The thumbnail size in DIP.
 const int kThumbnailWidth = 212;
-const int kThumbnailHeight = 132;
+const int kThumbnailHeight = 142;
 
 // True if thumbnail retargeting feature is enabled (Finch/flags).
 bool IsThumbnailRetargetingEnabled() {