Bug 1052793, part 2 - Shrink GC buffers during compartmental GCs. r=terrence

With part 1, many more GCs are compartmental, so it is no longer a
good indication that there is heavy activity. If we don't regularly
try to shrink GC buffers, we don't decommit arenas enough, causing a
large regression in memory usage.

dom/base/nsJSEnvironment.cpp

@@ -2288,13 +2288,10 @@ DOMGCSliceCallback(JSRuntime *aRt, JS::GCProgress aProgress, const JS::GCDescrip
         }
       } else {
         nsJSContext::KillFullGCTimer();
+      }
 
-        // Avoid shrinking during heavy activity, which is suggested by
-        // compartment GC. We don't need to shrink after a shrinking GC as this
-        // happens automatically in this case.
-        if (aDesc.invocationKind_ == GC_NORMAL) {
-          nsJSContext::PokeShrinkGCBuffers();
-        }
+      if (aDesc.invocationKind_ == GC_NORMAL) {
+        nsJSContext::PokeShrinkGCBuffers();
       }
 
       if (ShouldTriggerCC(nsCycleCollector_suspectedCount())) {