Make sure DeviceRequestMessageFilter call MediaStreamManager::CancelRequest for all pending requests to MediaStreamManager::EnumerateDevices when the IPC channel is teared down.

Before this change DeviceRequestMessageFilter call CancelRequest with the render process pid. But that pid was not been used as an identifier when calling EnumerateDevices. 

BUG=338561

Review URL: https://codereview.chromium.org/144363005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247744 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247744