HttpCache: be more conservative when trying to reconstruct a full resource from partial one.

(With partial resource being a 206, not a truncated one).

Previously we would happily use a first chunk from cache, then fail in panic when validation
of the second chunk failed; this was especially bad for servers whose validation fails to
match for resources for which headers suggest it should.

After this change, in this case, we force a validation of the first chunk, and if it fails
re-fetch the entire resource. This is far from ideal, but we have limitations when it comes
to converting a pre-existing 206 entry to a 200 one that make using somewhat-better-in-this-case
If-Range really painful (and there is no multi-range support, that would be useful for these
sorts of reconstructions, either).

(Heavily affects the lazyload experiment)

Bug: 888742
Change-Id: I7f3f3b40dc1e2a98dd093ac09c98fe43eee5f3bd
Reviewed-on: https://chromium-review.googlesource.com/c/1291595
Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606092}

• net/http/http_cache_transaction.cc[diff]
• net/http/http_cache_unittest.cc[diff]
• net/http/partial_data.cc[diff]
• net/http/partial_data.h[diff]