Fix races when the same bits are downloaded from 2 URLs.

Until now, we were saving downloaded file to the temporary directory
with a name being the hash of the file content. It means there is a race
when the same content is downloaded from 2 different URLs.

To fix this, we now create an intermediary directory that is the hash of
the URL.

Also, because this is only needed for debugging with gdb, and this is
inefficient in term of both CPU and storage (we do not know when to
delete the temporary directory), we control this with a command line
flag.

R=ncbray@chromium.org, eseidel@chromium.org
BUG=https://github.com/domokit/mojo/issues/61

Review URL: https://codereview.chromium.org/1011333003

tools/skydb

@@ -131,6 +131,7 @@ class SkyDebugger(object):
 
         if is_android and args.gdb:
             shell_args.append('--wait-for-debugger')
+            shell_args.append('--predictable-app-filenames')
 
         if 'remote_sky_server_port' in self.pids:
             shell_command = self._wrap_for_android(shell_args)