Fixes use after free in ContentHandlerImpl

Current code assumes the callback is run before the thread is
joined. That isn't necessarily the case though. It's entirely possible
for the callback to queued up, then join to complete. When this
happens and the callback gets run the ContentHandlerImpl has since
been deleted.

This use after free came up in trying to get asan running on our
tests.

BUG=438727
TEST=none
R=qsr@chromium.org

Review URL: https://codereview.chromium.org/777763002