Disable warning in leveldb

Patch has been committed upstream (internal) but it will be a while before it's
released to code.google.com, and then rolled into Chromium. So, disable
minor warning for now.

R=thakis@chromium.org
BUG=126483

Review URL: https://chromiumcodereview.appspot.com/10384092

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@137678 0039d316-1c4b-4281-b951-d872f2087c98

third_party/leveldatabase/leveldatabase.gyp

@@ -55,6 +55,13 @@
             'include_dirs': [
               'src/port/win',
             ],
+            # Patch posted for upstream, can be removed once that's landed and
+            # rolled into Chromium.
+            # Internal link: https://mondrian.corp.google.com/#review/29997992
+            'msvs_disabled_warnings': [
+              # Signed/unsigned comparison.
+              4018,
+            ],
           }],
         ],
       },