[Wallpaper Picker] Added experimental permission for transparent background

So that users don't have to manually enable the "experimental extension APIs" flag, I think it's OK to add the "experimental" permission to have a nice transparent background for everyone like the Files App does. See http://src.chromium.org/svn/trunk/src/chrome/browser/resources/file_manager/manifest.json

Once http://crbug.com/262345 is fixed, we'll be able to remove the experimental permissions from the Wallpaper Picker and the Files App. 

Screenshot: http://i.imgur.com/dTbJOJh.png

Review URL: https://codereview.chromium.org/182833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253862 0039d316-1c4b-4281-b951-d872f2087c98
Gracefully fall back to a reasonable voice when a non-existent voice name is supplied.

BUG=346709

Review URL: https://codereview.chromium.org/176923014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253863 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253863