Move WinPrecompile out of Source/WebKit/chromium

We're in the process of deleting Source/WebKit/chromium, which means we need to
find new homes for all these files. This CL moves the Windows precompiled
header into a new Source/build/win directory.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/21135002

git-svn-id: svn://svn.chromium.org/blink/trunk@155305 bbb929c8-8fbe-4397-9dbb-9b2b20218538