port 24a7ee877e1fe2ba0a52d69061946aeda7c26c10 (r25297)

original commit message:

  Leaving a generator via an exception causes it to close

BUG=
R=weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/724643002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

Cr-Commit-Position: refs/heads/master@{#25316}