[autotest] Add retry to gmail_lib

Retry on the error 429: User-rate limit exceeded

TEST=python gmail_lib.py -s 'hello' fdeng@google.com
BUG=chromium:535616

Change-Id: I2e80514f718289effdafd28c58943c21b0f3021f
Reviewed-on: https://chromium-review.googlesource.com/302585
Commit-Ready: Fang Deng <fdeng@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
1 file changed