Profile how primitive runs / clip nodes are being produced by Gecko

[gw3583]

It would be really interesting to profile a series of real world pages in Gecko, and get a sense of:

• What the ratio of clip nodes is to primitives.
• How many primitives (on average) there are per primitive run in a Picture.

From this information, we may be able to either:

• Improve the display lists that Gecko is producing, if there are redundant clips (either in Gecko or in WR itself).
• See if there is a better way to store some of this information given the real world usage patterns we see.

[gw3583]

cc @mrobinson @kvark

[gw3583]

See also: #2765