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Currently for each download stream, the data read from the network
was immediately written to the disk.
This CL allows each SourceStream to read some data before the file
write offset for content validation purpose. And data will only be
written after all the initial validation data are consumed.
A new field called starting_file_write_offset_ is added to SourceStream
to identify this checkpoint for file validation.

BUG=965215

Change-Id: Ia46de1a588bc0c2fb4e6eb19e62ef794d706ad1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625461
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664030}