[snapshot] revisit snapshot API.

[snapshot] revisit snapshot API.

This part of the snapshot API should not be in use yet, so we can still
change this. The motivation for this change is:
- Use MaybeHandle where reasonable.
- Remove ambiguity: when we use index to create context from snapshot,
  we should not have a silent fallback if snapshot is not available.
- Symmetry: rename to Context::FromSnapshot to mirror templates.

R=jochen@chromium.org
BUG=chromium:617892
Committed: https://crrev.com/872c461b0003f0d453afcbd3c166faeb132293f8
Cr-Commit-Position: refs/heads/master@{#37334}

[Yang, 2016-06-28 06:06:32]

On 2016/06/27 07:46:09, Yang wrote:

friendly ping :)

[jochen (gone - plz use gerrit), 2016-06-28 13:12:59]

lgtm

[Yang, 2016-06-28 13:18:31]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2016-06-28 13:18:40]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-06-28 13:47:48]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-06-28 13:48:11]

Description was changed from

==========
[snapshot] revisit snapshot API.

This part of the snapshot API should not be in use yet, so we can still
change this. The motivation for this change is:
- Use MaybeHandle where reasonable.
- Remove ambiguity: when we use index to create context from snapshot,
  we should not have a silent fallback if snapshot is not available.
- Symmetry: rename to Context::FromSnapshot to mirror templates.

R=jochen@chromium.org
BUG=chromium:617892
==========

to

==========
[snapshot] revisit snapshot API.

This part of the snapshot API should not be in use yet, so we can still
change this. The motivation for this change is:
- Use MaybeHandle where reasonable.
- Remove ambiguity: when we use index to create context from snapshot,
  we should not have a silent fallback if snapshot is not available.
- Symmetry: rename to Context::FromSnapshot to mirror templates.

R=jochen@chromium.org
BUG=chromium:617892

Committed: https://crrev.com/872c461b0003f0d453afcbd3c166faeb132293f8
Cr-Commit-Position: refs/heads/master@{#37334}
==========

[commit-bot: I haz the power, 2016-06-28 13:48:12]

Patchset 1 (id:??) landed as
https://crrev.com/872c461b0003f0d453afcbd3c166faeb132293f8
Cr-Commit-Position: refs/heads/master@{#37334}