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This allows embedders to send a chunk when they do not yet know if it is
the final chunk or not.

BUG=405678

Review URL: https://codereview.chromium.org/492063002

Cr-Commit-Position: refs/heads/master@{#291222}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291222 0039d316-1c4b-4281-b951-d872f2087c98