stylo: Measure Elements and ComputedValues.

The patch provides FFI access to Gecko's SeenPtrs type from Rust, in
order to record what has already been measured when measuring Arcs. (The
SeenPtrs must be initialized on the Gecko side because the same table is
reused for measuring all Elements within a window, because Elements can
share ComputedValues.) I have confirmed with DMD that this is working
correctly.

The patch also introduces MallocSizeOfRepeats, which is like
MallocSizeOf but takes a SizeOfState, which holds a SeenPtrs table.

components/servo_arc/lib.rs

@@ -41,6 +41,7 @@ use std::hash::{Hash, Hasher};
 use std::iter::{ExactSizeIterator, Iterator};
 use std::mem;
 use std::ops::{Deref, DerefMut};
+use std::os::raw::c_void;
 use std::process;
 use std::ptr;
 use std::slice;
@@ -200,6 +201,7 @@ impl<T> Arc<T> {
     pub fn borrow_arc<'a>(&'a self) -> ArcBorrow<'a, T> {
         ArcBorrow(&**self)
     }
+
     /// Temporarily converts |self| into a bonafide RawOffsetArc and exposes it to the
     /// provided callback. The refcount is not modified.
     #[inline(always)]
@@ -218,6 +220,12 @@ impl<T> Arc<T> {
         // Forward the result.
         result
     }
+
+    /// Returns the address on the heap of the Arc itself -- not the T within it -- for memory
+    /// reporting.
+    pub fn heap_ptr(&self) -> *const c_void {
+        self.p.ptr() as *const ArcInner<T> as *const c_void
+    }
 }
 
 impl<T: ?Sized> Arc<T> {