Fix underflow in computation of PrivateMemoryFootprint on macOS.

The underflow was occurring during subtraction of two uints, before the
saturated_cast could fix the result.

Bug: 812346
Change-Id: I08acf91e137880fe7aba0d41f8ea362c9f81315c
Reviewed-on: https://chromium-review.googlesource.com/955908Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543533}

services/resource_coordinator/memory_instrumentation/queued_request_dispatcher.cc

@@ -46,15 +46,18 @@ uint32_t CalculatePrivateFootprintKb(const mojom::RawOSMemDump& os_dump,
   if (base::mac::IsAtLeastOS10_12()) {
     uint64_t phys_footprint_bytes =
         os_dump.platform_private_footprint->phys_footprint_bytes;
-    return base::saturated_cast<uint32_t>(phys_footprint_bytes / 1024 -
-                                          shared_resident_kb);
+    return base::saturated_cast<uint32_t>(
+        base::saturated_cast<int32_t>(phys_footprint_bytes / 1024) -
+        base::saturated_cast<int32_t>(shared_resident_kb));
   } else {
     uint64_t internal_bytes =
         os_dump.platform_private_footprint->internal_bytes;
     uint64_t compressed_bytes =
         os_dump.platform_private_footprint->compressed_bytes;
     return base::saturated_cast<uint32_t>(
-        (internal_bytes + compressed_bytes) / 1024 - shared_resident_kb);
+        base::saturated_cast<int32_t>((internal_bytes + compressed_bytes) /
+                                      1024) -
+        base::saturated_cast<int32_t>(shared_resident_kb));
   }
 #elif defined(OS_WIN)
   return os_dump.platform_private_footprint->private_bytes / 1024;