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Currently it's flaky that a partial buffer is read through network
pipeline when the first history db record is generated.

This CL changed the observer to be able to optionally wait for a
certain number of bytes read from network IO.

Bug: 788160
Change-Id: I110653433e8c80c2dabf9f3162df99d7bb354e86
Reviewed-on: https://chromium-review.googlesource.com/790946Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519930}