Preserve GYP_DEFINES when running layout tests on windows.
Preserve GYP_DEFINES when running layout tests on windows.

We try to get 'windows_sdk_path' from GYP_DEFINES to locate the winsdk when searching for cdb.exe

BUG=none
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/73643006

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@162309 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs