Fix the official Android builder's GN build.

I don't think this needs a sysroot, and compiles for ARM, so I removed the assert and use an empty sysroot. If we need the sysroot we'll discover that when we try to make a target later.

BUG=330722
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/109023006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242554 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242554