Change WebRTCInternals::GetInstance() to use a leaky LazyInstance.

Change WebRTCInternals::GetInstance() to use a leaky LazyInstance.

WebRTCInternals uses NotificationRegistrar which must be deleted on the UI thread. In cases where the UI thread is not the same as the main thread (like in Chromium Embedded Framework) the destruction of WebRTCInternals via AtExitManager at shutdown will cause a CalledOnValidThread() check failure. There's no particular reason why the object needs to be deleted at shutdown so just leak it instead.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/575283004

Cr-Commit-Position: refs/heads/master@{#295751}