Fix requests for audio and video permissions

Requesting MediaAudioCapture would unconditionally ask for
QQuickWebEngineViewExperimental::MediaAudioVideoDevices instead of
MediaAudioDevices since the if wrongly tested for the same condition
twice.

In addition fix a warning: variable 'feature' is used uninitialized whenever 'if' condition is false

api/qquickwebengineview.cpp:369:13: warning: variable 'feature' is used uninitialized whenever 'if' condition is false
      [-Wsometimes-uninitialized]
   else if (requestFlags.testFlag(WebContentsAdapterClient::MediaVideoCapture))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
api/qquickwebengineview.cpp:371:57: note: uninitialized use occurs here
   Q_EMIT e->featurePermissionRequested(securityOrigin, feature);
                                                        ^~~~~~~
api/qquickwebengineview.cpp:369:9: note: remove the 'if' if its condition is always true
   else if (requestFlags.testFlag(WebContentsAdapterClient::MediaVideoCapture))
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
api/qquickwebengineview.cpp:364:4: note: variable 'feature' is declared here
   QQuickWebEngineViewExperimental::Feature feature;
   ^

Change-Id: If63fbb5a59a71dede1d1829695f66c22849a4dcd
Reviewed-by: Michael Bruning <michael.bruning@digia.com>