Crash in supportsSnapshotting
https://bugs.webkit.org/show_bug.cgi?id=117670
<rdar://problem/14059711>

Reviewed by Darin Adler.

Speculative fix for deferencing a null RefPtr in supportsSnapshotting.
It seems like it was possible to get in a state where the plugin machinery
would fire up and attempt to get a screenshot before there was an active
connection to the plugin process (usually under periods of high load, such
as opening a lot of tabs at once). In this case we're now returning false
from supportsSnapshotting, which will trigger the plugin to restart, but
that's better than requesting a snapshot on something that doesn't yet exist.

* WebProcess/Plugins/PluginProxy.cpp:
(WebKit::PluginProxy::supportsSnapshotting): Guard for an empty connection object.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@151617 268f45cc-cd09-0410-ab3c-d52691b4dbfc