Build fix when use_alsa=0.

When use_alsa=0, there is a link error as follows:

../../media/audio/pulse/audio_manager_pulse.cc:81: error: undefined reference to 'media::AudioManagerAlsa::ShowLinuxAudioInputSettings()'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

This patch allows to add a guard to alsa related code to fix this link error.

BUG=none

Review URL: https://codereview.chromium.org/150573017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249361 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249361