Remove unneeded WEBKIT_IMPLEMENTATION #ifs on Platform.h header.

Reviewed by: Marcelo Lira
Remove #if WEBKIT_IMPLEMENTATION from WebGamePad headers.

Reviewers: setanta, Tullio, lmoura, dakerfp, lacerda, sergio, nick

Differential Revision: http://phabricator.coderev.indt.org/D2387
Remove "using namespace" from header to avoid side effects of unwanted global namespace polution.

Reviewed by: Marcelo Lira