linux: add -fvisibility=hidden to build flags

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index b5e22791e3f36f3b52ad5dca4ffb251a641b23bb..da571a965ef882527029b9c91ddf3254189f369b 100644
--- a/build/common.gypi
+++ b/build/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' ],
+          }],
         ],
       },
     }],