Revert "Move WinPrecompile out of Source/WebKit/chromium"

This reverts https://src.chromium.org/viewvc/blink?view=revision&revision=155117

Broke the Windows build.

TBR=abarth

Review URL: https://codereview.chromium.org/21193002

git-svn-id: svn://svn.chromium.org/blink/trunk@155123 bbb929c8-8fbe-4397-9dbb-9b2b20218538