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This is part of work to allow embedders to send a chunk when they do
not yet know whether it is the final chunk.

BUG=405678

Review URL: https://codereview.chromium.org/497553002

Cr-Commit-Position: refs/heads/master@{#291282}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291282 0039d316-1c4b-4281-b951-d872f2087c98