Update the gyp variable used to disable tcmalloc

This changed with chrome 37. We only need to define it unconditionally
in gyp_qtwebengine as tcmalloc is never an option for us.

Change-Id: I8b21eb1114582c1d9a8aa61c5f15fc8469721395
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>