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The logic to calculate how much data to write is getting bigger,
move to a separate function.
This CL also adds a check if the target location is already written.
If so, it will request the current stream to terminate.
Unit test will pass once https://codereview.chromium.org/2737033002/ lands

BUG=644352

Review-Url: https://codereview.chromium.org/2744793003
Cr-Commit-Position: refs/heads/master@{#457839}