Bug 1447951 - Store nsDynamicAtom's chars after the end of the object. r=froydnj

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.

parser/html/nsHtml5AtomTable.cpp

@@ -7,7 +7,7 @@
 
 nsHtml5AtomEntry::nsHtml5AtomEntry(KeyTypePointer aStr)
   : nsStringHashKey(aStr)
-  , mAtom(new nsDynamicAtom(*aStr))
+  , mAtom(nsDynamicAtom::Create(*aStr))
 {
 }
 
@@ -20,7 +20,7 @@ nsHtml5AtomEntry::nsHtml5AtomEntry(const nsHtml5AtomEntry& aOther)
 
 nsHtml5AtomEntry::~nsHtml5AtomEntry()
 {
-  delete mAtom;
+  nsDynamicAtom::Destroy(mAtom);
 }
 
 nsHtml5AtomTable::nsHtml5AtomTable()