Remove #if WEBKIT_IMPLEMENTATION from WebGamePad headers.

Reviewed by: Marcelo Lira
Remove "using namespace" from header to avoid side effects of unwanted global namespace polution.

Reviewed by: Marcelo Lira
[cosmetic] Just remove the unneeded comma.
Remove custom assert function from Nix::Platform and uses the standard one.

Reviewed by: Marcelo Lira
Remove windows stuff from WebCommon.h

We don't support Windows and there are no plans yet for such support,
so makes more sense to have a clean header.

Reviewed by: Marcelo Lira
Get rid of WEBKIT_IMPLAMENTATION in favor of the already BUILDING_NIX__

There are no more reasons to keep two defines with the same meaning.