Expand a comment to clarify why the threshold of 1077MB is used.

Bug: 843775
Change-Id: I3a3f8689369bd9b447e0254396a0326e0d0db88b
Reviewed-on: https://chromium-review.googlesource.com/1069566Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560845}

content/browser/renderer_host/render_process_host_impl.cc

@@ -2532,6 +2532,11 @@ bool RenderProcessHostImpl::IsSpareProcessKeptAtAllTimes() {
 
   // Spare renderer actually hurts performance on low-memory devices.  See
   // https://crbug.com/843775 for more details.
+  //
+  // The comparison below is using 1077 rather than 1024 because 1) this helps
+  // ensure that devices with exactly 1GB of RAM won't get included because of
+  // inaccuracies or off-by-one errors and 2) this is the bucket boundary in
+  // Memory.Stats.Win.TotalPhys2.
   if (base::SysInfo::AmountOfPhysicalMemoryMB() <= 1077)
     return false;