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about:memory statistics improvements

The three major memory costs to rendering webpages in Gecko are layout, the DOM, and JavaScript.  Only the last of these has a detailed about:memory breakdown.  Layout has a few subcategories, but likely as not, layout/arenas is one big opaque number, and DOM just sits as a big blob-o-stuff.

Over the past two weeks, I’ve been working to improve this state of affairs.  DOM’s numbers have been made more transparent by splitting out the numbers by the type of DOM node.  Admittedly, this isn’t perfect, as now you have several semi-opaque blobs, but it gives you a slightly better idea where the memory is going.  Additionally, a few more things are getting counted in those DOM numbers, like some on-the-side datastructures Gecko’s DOM engine keeps around.

Layout has received the most work, however.  We now display stats for individual frame types and common objects allocated in layout’s arenas, which can be helpful in diagnosing page performance problems (bug 686795 comment 10 and comment 23).  Please note that in the stats for these frames and objects, we’re not measuring any substructures contained in them, some of which could be quite significant.  That’s a (potentially tricky) job for later.