Fix Initialize & Dispose for external snapshot.

Fix Initialize & Dispose for external snapshot. Make sure v8::V8::(Initialize|Dispose) can be called in any order.

This is a follow-on to crrev.com/960883003, which fixed a memory leak in this code, but uncovered another, more subtle bug:

Previously, the code expected you would v8::V8::Initialize once, and v8::V8::Dispose once. The first bug was that in this case the holder_ variable would point to deallocated memory. The second bug was that once the snapshot was disposed, there was no way to get it back on a future Initialize. These are uncovered by the InitializeAndDisposeMultiple test case.

The fix is to keep memory to the raw snapshot and to then cleanly build & destroy the tables in Initialize & Dispose. Since sometimes setNativesBlob is called just after Initialize, that situation must be handled, too.

BUG=

Committed: https://crrev.com/2525e8f4026307cf7116e64bc0e9a1bb512cbfff
Cr-Commit-Position: refs/heads/master@{#26978}

[vogelheim, 2015-03-03 17:42:08]

vogelheim@chromium.org changed reviewers:
+ jochen@chromium.org

[jochen (gone - plz use gerrit), 2015-03-03 18:30:53]

lgtm

[vogelheim, 2015-03-04 10:36:21]

The CQ bit was checked by vogelheim@chromium.org

[commit-bot: I haz the power, 2015-03-04 10:36:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/974943003/1

[commit-bot: I haz the power, 2015-03-04 10:37:52]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 10:38:15]

Patchset 1 (id:??) landed as
https://crrev.com/2525e8f4026307cf7116e64bc0e9a1bb512cbfff
Cr-Commit-Position: refs/heads/master@{#26978}