Bug 645416, part 3 - Symbol layout and GC support for allocating them. r=terrence.

Layout: js/src/vm/Symbol.h defines the new class JS::Symbol. JS::Symbol is the
same size as JSString on all platforms, because the allocator does not support
smaller allocations.

Allocation: Since the purpose of symbols is to serve as property keys, they are
always allocated in the atoms compartment.

We take a lock when allocating. This could probably be replaced with a
main-thread-only assertion. However, if atom allocation is not already a
bottleneck, symbol allocation probably never will be.

Symbols are given their own finalize-class in the GC. This means we allocate a
page per zone for symbols, even though they are only ever allocated in the
atoms zone. Terrence thought this could be easily fixed later. It should be; we
never touch the page, but a 32-bit virtual address space does not just have
infinite pages to spare.

A jsapi-test exercises the new symbol allocation code. A few oddities in
jsapi-tests are fixed in passing.

Discussion after review led to some new assertions about minimum object size in
AllocateObject and AllocateNonObject.

xpcom/base/CycleCollectedJSRuntime.cpp

@@ -574,6 +574,7 @@ CycleCollectedJSRuntime::DescribeGCThing(bool aIsMarked, void* aThing,
     static const char trace_types[][11] = {
       "Object",
       "String",
+      "Symbol",
       "Script",
       "LazyScript",
       "IonCode",