Remove unnecessary copies of PlatformMediaSessionManager::m_sessions

Remove unnecessary copies of PlatformMediaSessionManager::m_sessions
https://bugs.webkit.org/show_bug.cgi?id=175375

Patch by Ms2ger <Ms2ger@igalia.com> on 2017-08-09
Reviewed by Sam Weinig.

Remove unnecessary copies of PlatformMediaSessionManager::m_sessions.

These should have been removed in r207688.

No new tests: no behavior change.

* platform/audio/PlatformMediaSessionManager.cpp:
(WebCore::PlatformMediaSessionManager::applicationWillBecomeInactive const):
(WebCore::PlatformMediaSessionManager::applicationDidBecomeActive const):
(WebCore::PlatformMediaSessionManager::applicationDidEnterBackground const):
(WebCore::PlatformMediaSessionManager::applicationWillEnterForeground const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220454 268f45cc-cd09-0410-ab3c-d52691b4dbfc