NTP4: Remove gradient while printing most visited thumbnails.

There is currently a bug in WebKit that renders -webkit-linear-gradient()
differently across platforms when trying to print (shows no gradient on
Linux/Win, shows gradient without alpha on Mac?), so we'll just remove the 
gradient when users are printing the Most Visited section of the NTP so they can 
see their pretty thumbnails.

Upstream WebKit bug:
https://bugs.webkit.org/show_bug.cgi?id=19942

R=estade@chromium.org
TBR=csilv@chromium.org
TEST=Print preview shows Most Visited tiles on all platforms.
BUG=103443

Review URL: http://codereview.chromium.org/8840001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113364 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/ntp4/most_visited_page.css

@@ -121,6 +121,13 @@ html[dir=rtl] .most-visited .close-button {
                                       rgba(255, 255, 255, 0.9));
 }
 
+/* TODO(dbeam): Remove this when printing of -webkit-linear-gradient() works. */
+@media print {
+  .thumbnail-shield {
+    background: none;
+  }
+}
+
 .most-visited:focus .thumbnail,
 .most-visited:hover .thumbnail {
   opacity: 0.95;