Bug 1376638 - Minimize uses of prmem.h. r=glandium.

It's silly to use prmem.h within Firefox code given that in our configuration
its functions are just wrappers for malloc() et al. (Indeed, in some places we
mix PR_Malloc() with free(), or malloc() with PR_Free().)

This patch removes all uses, except for the places where we need to use
PR_Free() to free something allocated by another NSPR function; in those cases
I've added a comment explaining which function did the allocation.

xpcom/io/nsLocalFileUnix.cpp

@@ -1908,7 +1908,7 @@ nsLocalFile::SetPersistentDescriptor(const nsACString& aPersistentDescriptor)
   AliasRecord aliasHeader = *(AliasPtr)decodedData;
   int32_t aliasSize = ::GetAliasSizeFromPtr(&aliasHeader);
   if (aliasSize > ((int32_t)dataSize * 3) / 4) { // be paranoid about having too few data
-    PR_Free(decodedData);
+    PR_Free(decodedData); // PL_Base64Decode() uses PR_Malloc().
     return NS_ERROR_FAILURE;
   }
 
@@ -1920,7 +1920,7 @@ nsLocalFile::SetPersistentDescriptor(const nsACString& aPersistentDescriptor)
   if (::PtrToHand(decodedData, &newHandle, aliasSize) != noErr) {
     rv = NS_ERROR_OUT_OF_MEMORY;
   }
-  PR_Free(decodedData);
+  PR_Free(decodedData); // PL_Base64Decode() uses PR_Malloc().
   if (NS_FAILED(rv)) {
     return rv;
   }