mac ninja/make: Cache _sdk_base_dir at class level instead of instance level.
mac ninja/make: Cache _sdk_base_dir at class level instead of instance level.

Makes gyp with GYP_GENERATORS=ninja file generation 20% faster.

BUG=82230
TEST=none
Review URL: https://chromiumcodereview.appspot.com/10451056
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google