This patch implements the window letterboxing. The implementation
is based on adding margins around the browser element to round the
content viewport size. Whenever the browser content is resized, the
RFPHelper will adjust margins around it. But it won't add any margins
for an empty browser or a browser loads a content with the system
principal.

The letterboxing is hidden behind a hidden pref
"privacy.resistFingerprinting.letterboxing." By default, it will use
stepping size 200x100 to round content window. And we can customize
the set of dimensions used for deciding the size of the rounded
content viewport by the pref
"privacy.resistFingerprinting.letterboxing.dimensions". This pref
should be formated as 'width1xheight1, width2xheight2, ...'. We will
find the dimensions which can fit into the real content size and have
the smallest margins to be the rounded content viewport size. For example
, given the set "400x200, 500x300, 800x500" and the real content size
"600x300", we would round the content size into 500x300.

--HG--
extra : rebase_source : a5bcb64934177f8518ef4bca922d7e3330bbd3ca