Clear response_pending_ in NotifyTokenCallbacks()

Every caller of OAuthTokenGetterImpl::NotifyTokenCallbacks() clears
this flag, so this CL moves it into the method.

Change-Id: Ib307c75afcfcadde1b0eb4a852650edd314383fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583103
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653889}
1 file changed