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- Splits Write into two parts, response time and data. Uses stream 0
  for response time, stream 1 for data, to avoid a slowdown.
- Splits Read similarly.
- Eliminates the extra data copy when writing caused by passing a
  span from CodeCacheHostImpl rather than the Mojo BigBuffer received
  from the renderer.
- Changes the keyed PendingOperation queues to keep the active op at
  the head. This simplifies the helper method and callback signatures.
- PendingOperations now own the IOBuffers for Write and Fetch.
- Using stream 0 now will make deduplication of the code caches (not
  storing a copy of the code for each origin) easier in the future.

Behavior changes:
- IssuePendingOperations now runs PendingOperations through the queues
  rather than all at once. This was probably didn't happen in the real
  world but seems incorrect.
- Reads where one or more parts fail now doom the entry. We should
  help the cache eliminate inaccessible data.
- Response time is now stored in stream 0. This means any data in the
  cache from previous Chromes where response time was a prefix on
  stream 1 will cause a cache miss, since the time stamp read will fail.
  This CL then makes the data read fail, since returning data prefixed
  with a timestamp could be confusing now. Invalidating the caches
  shouldn't be a problem, since we expect a new version of Chrome/V8
  to cause the code caches to become invalid since their version/feature
  header won't match, see:
  https://cs.chromium.org/chromium/src/v8/src/snapshot/code-serializer.cc?rcl=6c89d2ffb531b3c79181532b5de04adaf7206049&l=387

Bug: chromium:992991
Change-Id: If3c5e8a83bca811fcb809a8a89c6d22942456f13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814685
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700307}