Revert r156542 "Do not include "wtf/Platform.h" directly. "

This patch breaks win_x64_rel build:
http://build.chromium.org/p/tryserver.chromium/builders/win_x64_rel/builds/32474

BUG=none
TEST=none
TBR=podivilov@google.com

Review URL: https://codereview.chromium.org/23003024

git-svn-id: svn://svn.chromium.org/blink/trunk@156568 bbb929c8-8fbe-4397-9dbb-9b2b20218538