Store nsDynamicAtom's chars after the end of the object.

This reduces memory usage because we only need one allocation instead of two
for the dynamic atom and its chars, and because we don't need to store a
refcount and a size. It precludes sharing of chars between dynamic atoms, but
we weren't benefiting much from that anyway.

This reduces per-process memory usage by up to several hundred KiB on my
Linux64 box.

One consequence of this change is that we need to allocate + copy in
DOMString::SetKnownLiveAtom(), which could make some things slower.

Bug: 1447951
Reviewed-by: froydnj

components/style/gecko_string_cache/mod.rs

@@ -122,7 +122,8 @@ impl WeakAtom {
             unsafe { u8_ptr.offset(string_offset) as *const u16 }
         } else {
             let atom_ptr = self.as_ptr() as *const nsDynamicAtom;
-            unsafe { (*(atom_ptr)).mString }
+            // Dynamic atom chars are stored at the end of the object.
+            unsafe { atom_ptr.offset(1) as *const u16 }
         };
         unsafe { slice::from_raw_parts(string, self.len() as usize) }
     }