[serializer] Move WeakFixedArray compaction to separate heap walk phase

[serializer] Move WeakFixedArray compaction to separate heap walk phase

This avoids discovering and compacting the same array repeatedly. No change in resulting snapshots intended.

R=yangguo@chromium.org

Committed: https://crrev.com/c7ba2f7fa35f15d07750a50d6b6cb0cca98275e5
Cr-Commit-Position: refs/heads/master@{#30177}

[Jakob Kummerow, 2015-08-14 12:41:33]

As discussed.

For the Scripts' weak fixed array, it shouldn't matter when we clear them; I
moved clearing those as well for consistency.

[Yang, 2015-08-14 12:43:08]

On 2015/08/14 12:41:33, Jakob wrote:
> As discussed.
> 
> For the Scripts' weak fixed array, it shouldn't matter when we clear them; I
> moved clearing those as well for consistency.

lgtm.

[Jakob Kummerow, 2015-08-14 12:43:28]

The CQ bit was checked by jkummerow@chromium.org

[commit-bot: I haz the power, 2015-08-14 12:43:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1290393002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1290393002/1

[commit-bot: I haz the power, 2015-08-14 13:09:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-14 13:09:53]

Patchset 1 (id:??) landed as
https://crrev.com/c7ba2f7fa35f15d07750a50d6b6cb0cca98275e5
Cr-Commit-Position: refs/heads/master@{#30177}