Fix bytes vs str error in DownloadGerritHook.
Fix bytes vs str error in DownloadGerritHook.

It fails with: `write() argument must be str, not bytes`.

R=4606aec58cba917397ff2e4738f995e4758694f8@google.com

Change-Id: If6787140ea863ce3761a5c3747eb0b857b22fa47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3276505
Reviewed-by: Josip Sokcevic <4606aec58cba917397ff2e4738f995e4758694f8@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools