Don't try to access a NULL plugin element or document in PepperPluginInstanceImpl

Don't try to access a NULL plugin element or document in PepperPluginInstanceImpl

There appear to be cases where the WebElement or Document associated with a plugin
have been destroyed but the PepperPluginInstance isn't yet destroyed and there is
some attempt to script into the plugin which tries to access the plugin context.
In these case we should just return an empty handle to the context.

TBR=dmichael@chromium.org
BUG=411778
Committed: https://crrev.com/41d1beae12ad4f9ea055aea40fee8238490db07e
Cr-Commit-Position: refs/heads/master@{#293873}

[raymes, 2014-09-08 23:37:14]

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

[raymes, 2014-09-08 23:37:45]

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

[Sam McNally, 2014-09-08 23:47:42]

lgtm

[raymes, 2014-09-08 23:48:53]

The CQ bit was checked by raymes@chromium.org

[commit-bot: I haz the power, 2014-09-08 23:50:51]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/raymes@chromium.org/558513002/20001

[dmichael (off chromium), 2014-09-09 00:04:09]

lgtm

[commit-bot: I haz the power, 2014-09-09 06:34:08]

Committed patchset #2 (id:20001) as 72a60f5e9435d0b56393cdbf0c3092ca6252b93c

[commit-bot: I haz the power, 2014-09-10 03:51:18]

Patchset 2 (id:??) landed as
https://crrev.com/41d1beae12ad4f9ea055aea40fee8238490db07e
Cr-Commit-Position: refs/heads/master@{#293873}