Avoid clearing pointers on layers still in the mapping's vector of layers.

Avoid clearing pointers on layers still in the mapping's vector of layers.

When adjusting layers, a particular layer may be added to a grouped
mapping at a location earlier than where it already appears in the
layer vector. When we then clear the unused entries in the list, we
remove the layer and null its group mapping, despite it still being
present earlier in the vector.

R=vollick
BUG=507988
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=198534

[Stephen Chennney, 2015-07-08 18:46:59]

The clusterfuzz example on this is extremely nasty, so no new test.

[Stephen Chennney, 2015-07-08 18:47:43]

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

[commit-bot: I haz the power, 2015-07-08 18:48:12]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1227763002/1

[Ian Vollick, 2015-07-08 19:23:23]

On 2015/07/08 18:48:12, commit-bot: I haz the power wrote:
> Dry run: CQ is trying da patch. Follow status at
>  https://chromium-cq-status.appspot.com/patch-status/1227763002/1

lgtm, but please update the bug.

[commit-bot: I haz the power, 2015-07-08 19:48:12]

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

[commit-bot: I haz the power, 2015-07-08 19:48:12]

Dry run: This issue passed the CQ dry run.

[Stephen Chennney, 2015-07-08 20:34:20]

The CQ bit was checked by schenney@chromium.org

[commit-bot: I haz the power, 2015-07-08 20:34:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1227763002/1

[commit-bot: I haz the power, 2015-07-08 20:38:29]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=198534