Let gyp_chromium set GYP_CROSSCOMPILE for android and ios.

Then envsetup doesn't need to set it, and the chromium.gyp_env files
used in the ios build no longer has to set this either.

BUG=330631
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/175683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252649 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252649