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The initial iOS version was over-ambitious, and doesn't yet build. To
get the build working, this:
- only builds base:base on iOS, not base:*, and
- disables warnings-as-errors in third-party code for iOS, since
  at least NSS and ICU have warnings for iOS.

This also adds nss.gyp to the iOS build, now that its gyp support
has landed

BUG=None
TEST=None


Review URL: https://chromiumcodereview.appspot.com/10704153

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@146120 0039d316-1c4b-4281-b951-d872f2087c98