linux: add -fvisibility=hidden to build flags

We use this on OS X, so it shouldn't be harmful and it will fix a visibility
issue that came up in
  http://groups.google.com/group/chromium-dev/browse_thread/thread/800bda968bdc8f29

BUG=17943

Review URL: http://codereview.chromium.org/332035

git-svn-id: http://src.chromium.org/svn/trunk/src/build@30253 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -493,6 +493,7 @@
           '<(werror)',  # See note above about the werror variable.
           '-pthread',
           '-fno-exceptions',
+          '-fvisibility=hidden',
           '-Wall',
           '-D_FILE_OFFSET_BITS=64',
         ],
@@ -694,6 +695,11 @@
             'cflags': [ '-gstabs' ],
             'defines': ['USE_LINUX_BREAKPAD'],
           }],
+          ['library=="shared_library"', {
+            # When building with shared libraries, remove the visiblity-hiding
+            # flag.
+            'cflags!': [ '-fvisibility=hidden' ],
+          }],
         ],
       },
     }],