Rely on the map being a dictionary map rather than not having a backpointer

Rely on the map being a dictionary map rather than not having a backpointer

BUG=chromium:500173
LOG=n

Committed: https://crrev.com/72cdb99346d39a01536c7a2cc373ec250b01151e
Cr-Commit-Position: refs/heads/master@{#29074}

[Toon Verwaest, 2015-06-17 09:02:47]

verwaest@chromium.org changed reviewers:
+ jkummerow@chromium.org

[Toon Verwaest, 2015-06-17 09:02:48]

PTAL

[Jakob Kummerow, 2015-06-17 09:03:37]

LGTM. How about checking in the repro as a test?

[Toon Verwaest, 2015-06-17 09:09:59]

The CQ bit was checked by verwaest@chromium.org

[commit-bot: I haz the power, 2015-06-17 09:10:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194513003/20001

[commit-bot: I haz the power, 2015-06-17 09:27:39]

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

[commit-bot: I haz the power, 2015-06-17 09:27:41]

Try jobs failed on following builders:
  v8_linux64_avx2_rel on tryserver.v8 (JOB_FAILED,
http://build.chromium.org/p/tryserver.v8/builders/v8_linux64_avx2_rel/builds/...)

[Toon Verwaest, 2015-06-17 09:44:48]

The CQ bit was checked by verwaest@chromium.org

[Toon Verwaest, 2015-06-17 09:44:48]

The patchset sent to the CQ was uploaded after l-g-t-m from
jkummerow@chromium.org
Link to the patchset: https://codereview.chromium.org/1194513003/#ps40001
(title: "Don't cache dictionary changes")

[commit-bot: I haz the power, 2015-06-17 09:44:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1194513003/40001

[commit-bot: I haz the power, 2015-06-17 10:13:54]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-06-17 10:14:08]

Patchset 3 (id:??) landed as
https://crrev.com/72cdb99346d39a01536c7a2cc373ec250b01151e
Cr-Commit-Position: refs/heads/master@{#29074}