Fix the -fvisibility=hidden settings to work with the linux shlib build.
Fix the -fvisibility=hidden settings to work with the linux shlib build.

Inside chromium's src/build/common.gypi, when a shared library build is
specified, the -fvisibility=hidden flag is explicitly removed via a
cflags! exclusion.  This overrides all cflags set in lower gyp files. It
is also incorrect for building plugins.  To get around this, we use
a gyp pattern list so that the -fvisibility=hidden flag is readded by gyp
after the exclusion is applied.

-fvisibility=hidden is particularly important here because Module::Get()
has a second definition inside chromium that is used by the internal
pepper plugins.  Without -fvisibility=hidden, the symbol resolution
gets crossed after the plugin is loaded such that the browser internal
verison of Module::Get() is invoked by ppapi's C++ wrappers even though
the it is the plugin's version of Module::Get() that is initialized.
This usually manifests itself as all externally loaded ppapi plugins that
use the ppapi C++ wrappers failing to load.
Review URL: http://codereview.chromium.org/3018003
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