Fix sys.platform mapping in download_from_google_storage.py.

Relevant documentation: https://docs.python.org/3/library/sys.html#sys.platform

Bug: 1007872
Change-Id: Id4bb6e1da1d3244fcd245569fd74fd204ce1e9ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1824843
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

download_from_google_storage.py

@@ -38,7 +38,8 @@ GSUTIL_DEFAULT_PATH = os.path.join(
 PLATFORM_MAPPING = {
     'cygwin': 'win',
     'darwin': 'mac',
-    'linux2': 'linux',
+    'linux': 'linux',  # Python 3.3+.
+    'linux2': 'linux',  # Python < 3.3 uses "linux2" / "linux3".
     'win32': 'win',
     'aix6': 'aix',
     'aix7': 'aix',