Bug 1299384 - Use MOZ_MUST_USE with NS_warn_if_impl(). r=erahm.

This change avoids lots of false positives for Coverity's CHECKED_RETURN
warning, caused by NS_WARN_IF's current use in both statement-style and
expression-style.

In the case where the code within the NS_WARN_IF has side-effects, I made the
following change.

> NS_WARN_IF(NS_FAILED(FunctionWithSideEffects()));
> -->
> Unused << NS_WARN_IF(NS_FAILED(FunctionWithSideEffects()));

In the case where the code within the NS_WARN_IF lacks side-effects, I made the
following change.

> NS_WARN_IF(!condWithoutSideEffects);
> -->
> NS_WARNING_ASSERTION(condWithoutSideEffects, "msg");

This has two improvements.
- The condition is not evaluated in non-debug builds.
- The sense of the condition is inverted to the familiar "this condition should
  be true" sense used in assertions.

A common variation on the side-effect-free case is the following.

> nsresult rv = Fn();
> NS_WARN_IF_(NS_FAILED(rv));
> -->
> DebugOnly<nsresult rv> = Fn();
> NS_WARNING_ASSERTION(NS_SUCCEEDED(rv), "Fn failed");

xpcom/base/CycleCollectedJSRuntime.cpp

@@ -64,6 +64,7 @@
 #include "mozilla/Telemetry.h"
 #include "mozilla/TimelineConsumers.h"
 #include "mozilla/TimelineMarker.h"
+#include "mozilla/Unused.h"
 #include "mozilla/DebuggerOnGCRunnable.h"
 #include "mozilla/dom/DOMJSClass.h"
 #include "mozilla/dom/ProfileTimelineMarkerBinding.h"
@@ -817,10 +818,11 @@ CycleCollectedJSRuntime::GCSliceCallback(JSContext* aContext,
 
   if (aProgress == JS::GC_CYCLE_END) {
     JS::gcreason::Reason reason = aDesc.reason_;
-    NS_WARN_IF(NS_FAILED(DebuggerOnGCRunnable::Enqueue(aContext, aDesc)) &&
-               reason != JS::gcreason::SHUTDOWN_CC &&
-               reason != JS::gcreason::DESTROY_RUNTIME &&
-               reason != JS::gcreason::XPCONNECT_SHUTDOWN);
+    Unused <<
+      NS_WARN_IF(NS_FAILED(DebuggerOnGCRunnable::Enqueue(aContext, aDesc)) &&
+                 reason != JS::gcreason::SHUTDOWN_CC &&
+                 reason != JS::gcreason::DESTROY_RUNTIME &&
+                 reason != JS::gcreason::XPCONNECT_SHUTDOWN);
   }
 
   if (self->mPrevGCSliceCallback) {