An exprimental fix for Bug 65940.

It seems these leaks are caused by a reference-count mismatch for a ChannelProxy::Context object. (This class expects to increase its reference count in OnChannelOpened() and to decrease it in OnChannelClosed(). On the other hand, we cannot call OnChannelClosed() after calling ClearIPCMessageLoop() because it detaches the message loop of a ChannelProxy::Context object. That is, calling ChannelProxy::Close() in ChannelProxy::~ChannelProxy() cannot decrease the reference count to its context.) This change just explicitly calls ChannelProxy::Close() before calling ChannelProxy::ClearIPCMessageLoop().

BUG=65940
TEST=make the valgrind bot green without suppressions.
Review URL: http://codereview.chromium.org/5706001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@68831 0039d316-1c4b-4281-b951-d872f2087c98