Fixes possible use after free in WebLayerTreeViewImpl

Fixes possible use after free in WebLayerTreeViewImpl

WebLayerTreeViewImpl needs to destroy LayerTreeHost early on as
otherwise it's possible for the compositor thread to be accessing
WebLayerTreeViewImpl while WebLayerTreeViewImpl is part way through
the destructor and in a bad state.

BUG=none
TEST=none
R=fsamuel@chromium.org

Committed: https://crrev.com/918daff46650a7b3605fdb6c3c72b5ec3bfa687e
Cr-Commit-Position: refs/heads/master@{#339785}

[Fady Samuel, 2015-07-21 23:28:22]

lgtm

[sky, 2015-07-21 23:29:35]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2015-07-21 23:31:08]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1247803006/1

[commit-bot: I haz the power, 2015-07-21 23:53:07]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-21 23:54:06]

Patchset 1 (id:??) landed as
https://crrev.com/918daff46650a7b3605fdb6c3c72b5ec3bfa687e
Cr-Commit-Position: refs/heads/master@{#339785}