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Currently chrome reports HTTP response code to UMA and it always -1 when
the HTTP connection is failed.
The CL let chrome report net status error instead if the response code
is -1.

BUG=539608
TEST=Sync multiple files in Files.app.

Review-Url: https://codereview.chromium.org/2163903002
Cr-Commit-Position: refs/heads/master@{#408926}