Bug 1297658 - Avoid unnecessary checking in memory reporters. r=erahm.

This patch removes checking of all the callback calls in memory reporter
CollectReport() functions, because it's not useful.

The patch also does some associated clean-up.

- Replaces some uses of nsIMemoryReporterCallback with the preferred
  nsIHandleReportCallback typedef.

- Replaces aCallback/aCb/aClosure with aHandleRepor/aData for CollectReports()
  parameter names, for consistency.

- Adds MOZ_MUST_USE/[must_use] in a few places in nsIMemoryReporter.idl.

- Uses the MOZ_COLLECT_REPORT macro in all suitable places.

Overall the patch reduces code size by ~300 lines and reduces the size of
libxul by about 37 KiB on my Linux64 builds.

toolkit/components/url-classifier/nsUrlClassifierPrefixSet.cpp

@@ -277,12 +277,13 @@ nsUrlClassifierPrefixSet::CollectReports(nsIHandleReportCallback* aHandleReport,
   // No need to get mLock here because this function does not directly touch
   // the class's data members. (SizeOfIncludingThis() will get mLock, however.)
 
-  size_t amount = SizeOfIncludingThis(UrlClassifierMallocSizeOf);
-
-  return aHandleReport->Callback(
-    EmptyCString(), mMemoryReportPath, KIND_HEAP, UNITS_BYTES, amount,
+  aHandleReport->Callback(
+    EmptyCString(), mMemoryReportPath, KIND_HEAP, UNITS_BYTES,
+    SizeOfIncludingThis(UrlClassifierMallocSizeOf),
     NS_LITERAL_CSTRING("Memory used by the prefix set for a URL classifier."),
     aData);
+
+  return NS_OK;
 }
 
 size_t