Remove device cache for MediaStreamDispatcher::EnumerateDevices and simplify.
Remove unused callback EnumerationFailed.
The device cache was not used by anyone- pepper call StopEnumerateDevice after each call to EnumerateDevices.
Also the cache will be invalid as soon as the patch for creating device ids base on the security origin is relanded. (https://codereview.chromium.org/66803007/)

TEST= Make sure (on Crome build, not Chromium) youtube.com/my_webcam?privacy=public can acces webcam and microphone. Test in multiple tabs.
BUG=269139

Review URL: https://codereview.chromium.org/72603004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238116 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238116