Add filtering rules for Tizen

Filter files suffixed with _tizen or tizen/ directories, depending on
whenever 'tizen' or 'tizen_mobile' is set.For now on, let's stop using
conditionals for source code in our build system.

The rules will automatically apply to every project file, because common.gypi is
included by gyp_xwalk script.
Remove platform conditionals for source code

Now that we have file filter rules for Tizen, we don't need to use conditionals
on the build system for source code we own.
Disable gyp's circular reference checking for Mac

We have now a few false positives because the browser tests depends
on the runtime target which is currently declared in xwalk.gyp which
includes xwalk_tests.gypi which depends on the browser tests.

The target relationship is not circular, but the file references are.
[Extensions] Make the browsertests build on Mac

Fix all the nits that are making clang sad.
[Extensions] Make Mac build the external extensions samples

Do not strip the binaries because Mac tools don't like when you
change the default 'product_dir'.
Remove a platform conditional build

Using file suffixes instead.
Ident xwalk_tests.gypi properly