[MD Bookmarks] Add overflow: hidden to main container.

Bug: 807216
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2b2d3a0b9c8b3b1f74fae4c38bded90022485f33
Reviewed-on: https://chromium-review.googlesource.com/895223Reviewed-by: Patti <patricialor@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533223}

chrome/browser/resources/md_bookmarks/app.html

@@ -30,6 +30,7 @@
         display: flex;
         flex-direction: row;
         flex-grow: 1;
+        overflow: hidden;
       }
 
       #splitter {