Unflake plugins/attach-during-destroy.html and plugins/destroy-reentry.html

BUG=380167
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/316313002

git-svn-id: svn://svn.chromium.org/blink/trunk@175617 bbb929c8-8fbe-4397-9dbb-9b2b20218538