Let GYP recognize the 'WDK_DIR' environment variable when generating ninja output on Windows.
Let GYP recognize the 'WDK_DIR' environment variable when generating ninja output on Windows.

TEST=Build chrome.exe using the compiler from Windows SDK 7.1

Review URL: https://chromiumcodereview.appspot.com/10377053

old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google