NativeContext::map_cache reference should be strong in heap snapshots

NativeContext::map_cache reference should be strong in heap snapshots

This change makes the reference itself a strong one. We will also need a special handling to make references from MapCache object to Map weak in the heap snapshots. The latter will be addressed separately.

BUG=chromium:357060
LOG=Y
R=alph@chromium.org, hpayer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=20304

[alph, 2014-03-27 12:58:22]

lgtm

[Hannes Payer (out of office), 2014-03-27 13:38:57]

lgtm

[yurys, 2014-03-27 14:10:59]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2014-03-27 14:11:03]

CQ is trying da patch. Follow status at
https://v8-status.appspot.com/cq/yurys@chromium.org/214683002/1

[commit-bot: I haz the power, 2014-03-27 14:11:04]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-03-27 14:11:04]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a lowly provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[yurys, 2014-03-27 14:19:58]

Committed patchset #1 manually as r20304 (presubmit successful).