[Tizen] Make the location provider implementation self-contained.

One of the problems with ld.bfd described in 7b83793 ("[Tizen] Start
using binutils-gold for linking Crosswalk again") was that when building
Crosswalk with -Dcomponent=shared_library ld.bfd would complain that
libcontent.so was missing symbol content::NewSystemLocationProvider().

Even though ld.gold links our binaries correctly, we are still having
problems _running_ them because the runtime linker cannot find an
implementation for that symbol (gyp causes lib/libcontent.so to be
passed after the .a archive containing the implementation in
content::NewSystemLocationProvider() from Crosswalk).

We can solve this the right way by making use of the infrastructure that
has been present in Chromium since at least M35 (!!): just override
ContentClientBrowser::OverrideSystemLocationProvider() and instantiate
Tizen's implementation there.
aren't we already in "xwalk" namespace?
return new LocationProviderTizen();