BatteryStatus: use 'unrestricted double' instead of 'double'

Even though it seems to be the same as far as our WebIDL
parser is concerned, it might be better to keep the IDL as
close to the specification as we can.

BUG=122593
TBR=haraken

Review URL: https://codereview.chromium.org/203683002

git-svn-id: svn://svn.chromium.org/blink/trunk@173079 bbb929c8-8fbe-4397-9dbb-9b2b20218538