HTTP cache: Add tests to improve coverage of AddTruncatedFlag.

There's also a small change in the logic that sets done_reading_, which
is used to determine if the truncated flag should be set or not, and should
not be used for sparse entries.

BUG=

Review URL: https://codereview.chromium.org/1346793002

Cr-Commit-Position: refs/heads/master@{#349528}
2 files changed