Check if the RenderView is alive before setting the plugin find handler

Check if the RenderView is alive before setting the plugin find handler

Presumably during destruction, the RenderView can be destroyed before
the RenderFrame (since it only holds a weak pointer to it). We need
to check whether it is alive before clearing the plugin find handler.

BUG=487146
Committed: https://crrev.com/0926126d8735958202f2201350e6cdec16357fc8
Cr-Commit-Position: refs/heads/master@{#330320}

[raymes, 2015-05-15 05:57:02]

raymes@chromium.org changed reviewers:
+ bbudge@chromium.org

[bbudge, 2015-05-15 16:04:16]

LGTM
Hopefully this check only needs to be done during destruction of the plugin.

[raymes, 2015-05-18 00:15:13]

The CQ bit was checked by raymes@chromium.org

[commit-bot: I haz the power, 2015-05-18 00:15:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1123393004/1

[commit-bot: I haz the power, 2015-05-18 01:13:19]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 11:33:19]

Patchset 1 (id:??) landed as
https://crrev.com/0926126d8735958202f2201350e6cdec16357fc8
Cr-Commit-Position: refs/heads/master@{#330320}