GitLab

On Windows, message pump can re-enter. AsyncHandleWaiter should
be prepared for that.

R=viettrungluu@chromium.org
BUG=none
TEST=ExtensionApiTest.SharedModuleWhitelist (this hits the DCHECK)

Review URL: https://codereview.chromium.org/973213002

Cr-Commit-Position: refs/heads/master@{#319202}