Prevent passed in LDFLAGS from trashing gyp-configured flags.
Prevent passed in LDFLAGS from trashing gyp-configured flags.

I missed this LDFLAGS -> GYP_LDFLAGS update in r768.
Review URL: http://codereview.chromium.org/500074
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp