Debugger: PreservePositionScope should clear positions inside the scope.

Debugger: PreservePositionScope should clear positions inside the scope.

The point of this change is so that when emitting code for a call in
FullCodegen::VisitCall, the statement position is not associated to
any code that loads the function, but to the actual CallIC.

R=mvstanton@chromium.org
BUG=chromium:481896
LOG=N

Committed: https://crrev.com/3f223ee69bdf66c31a1f0b340a8124834b82f93d
Cr-Commit-Position: refs/heads/master@{#28701}

[Yang, 2015-05-29 07:23:16]

The CQ bit was checked by yangguo@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-05-29 07:23:22]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1157543004/20001

[mvstanton, 2015-05-29 07:37:10]

lgtm

[commit-bot: I haz the power, 2015-05-29 07:48:05]

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

[commit-bot: I haz the power, 2015-05-29 07:48:06]

Dry run: This issue passed the CQ dry run.

[Yang, 2015-05-29 12:55:10]

The CQ bit was checked by yangguo@chromium.org

[commit-bot: I haz the power, 2015-05-29 12:55:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1157543004/20001

[commit-bot: I haz the power, 2015-05-29 12:56:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-05-29 12:56:52]

Patchset 2 (id:??) landed as
https://crrev.com/3f223ee69bdf66c31a1f0b340a8124834b82f93d
Cr-Commit-Position: refs/heads/master@{#28701}