Fold wtf/Platform.h into config.h and consolidate all global includes into one header.

wtf/Platform.h is only included from config.h. It no longer adds value to split the global includes into 2 header files.

Review URL: https://chromiumcodereview.appspot.com/23112015

git-svn-id: svn://svn.chromium.org/blink/trunk@156660 bbb929c8-8fbe-4397-9dbb-9b2b20218538