gn build: Let get.py keep zip file in memory instead of using a temp file

[thakis]

Summary

The zip is small, and it's a bit less code this way.
No intended behavior change.

[phosek]

LGTM

llvm/utils/gn/get.py

 #!/usr/bin/env python
 """Downloads a prebuilt gn binary to a place where gn.py can find it."""
 
 from __future__ import print_function
 
+import io
 import os
 import urllib2
 import sys
-import tempfile
 import zipfile
 
 
-def download_url(url, output_file):
-    """Download url into output_file."""
+def download_and_unpack(url, output_dir, gn):
+    """Download an archive from url and extract gn from it into output_dir."""
     print('downloading %s ...' % url, end='')
     sys.stdout.flush()
-    output_file.write(urllib2.urlopen(url).read())
+    data = urllib2.urlopen(url).read()
     print(' done')
-
-
-def download_and_unpack(url, output_dir, gn):
-    """Download an archive from url and extract gn from it into output_dir."""
-    with tempfile.TemporaryFile() as f:
-        download_url(url, f)
-        f.seek(0)
-        zipfile.ZipFile(f).extract(gn, path=output_dir)
+    zipfile.ZipFile(io.BytesIO(data)).extract(gn, path=output_dir)
 
 
 def set_executable_bit(path):
     mode = os.stat(path).st_mode
     mode |= (mode & 0o444) >> 2 # Copy R bits to X.
     os.chmod(path, mode) # No-op on Windows.