Update the <canvas> IDLs wrt 'unrestricted'

With the basic support for 'unrestricted' from
https://codereview.chromium.org/257783003 its possible to turn 'float'
into 'unrestricted float' where the spec has the attribute/parameter
marked that way.
Also apply 'unrestricted' to all the non-standard APIs.
When the actual support has landed it should be possible to remove any
explicit throwing as well, so mark such locations with FIXMEs.

BUG=367648

Review URL: https://codereview.chromium.org/257853002

git-svn-id: svn://svn.chromium.org/blink/trunk@172733 bbb929c8-8fbe-4397-9dbb-9b2b20218538