[sanitizer] Allow BackgroundThread to not depend on StackDepot v2

[cryptoad]

Summary

This is a redo of D45296.
It looks like the random stack-protector issues I was getting were coming from
my Android emulator, and updating everything all around and relaunching stuff
ended up making it go away.
I guess I'll have to see how it behaves on the bots.
Only additional change from the previous CL is some const were appropriate.

lib/sanitizer_common/sanitizer_common_libcdep.cc

 
 #include "sanitizer_common.h"
 
 #include "sanitizer_allocator_interface.h"
 #include "sanitizer_file.h"
 #include "sanitizer_flags.h"
 #include "sanitizer_procmaps.h"
 #include "sanitizer_report_decorator.h"
-#include "sanitizer_stackdepot.h"
 #include "sanitizer_stacktrace.h"
 #include "sanitizer_symbolizer.h"
 
 #if SANITIZER_POSIX
 #include "sanitizer_posix.h"
 #include <sys/mman.h>
 #endif
 

 
 static void (*SoftRssLimitExceededCallback)(bool exceeded);
 void SetSoftRssLimitExceededCallback(void (*Callback)(bool exceeded)) {
   CHECK_EQ(SoftRssLimitExceededCallback, nullptr);
   SoftRssLimitExceededCallback = Callback;
 }
 
 #if SANITIZER_LINUX && !SANITIZER_GO
+// Weak default implementation for when sanitizer_stackdepot is not linked in.
+SANITIZER_WEAK_ATTRIBUTE StackDepotStats *StackDepotGetStats() {
+  return nullptr;
+}
+
 void BackgroundThread(void *arg) {
-  uptr hard_rss_limit_mb = common_flags()->hard_rss_limit_mb;
-  uptr soft_rss_limit_mb = common_flags()->soft_rss_limit_mb;
-  bool heap_profile = common_flags()->heap_profile;
+  const uptr hard_rss_limit_mb = common_flags()->hard_rss_limit_mb;
+  const uptr soft_rss_limit_mb = common_flags()->soft_rss_limit_mb;
+  const bool heap_profile = common_flags()->heap_profile;
   uptr prev_reported_rss = 0;
   uptr prev_reported_stack_depot_size = 0;
   bool reached_soft_rss_limit = false;
   uptr rss_during_last_reported_profile = 0;
   while (true) {
     SleepForMillis(100);
-    uptr current_rss_mb = GetRSS() >> 20;
+    const uptr current_rss_mb = GetRSS() >> 20;
     if (Verbosity()) {
       // If RSS has grown 10% since last time, print some information.
       if (prev_reported_rss * 11 / 10 < current_rss_mb) {
         Printf("%s: RSS: %zdMb\n", SanitizerToolName, current_rss_mb);
         prev_reported_rss = current_rss_mb;
       }
       // If stack depot has grown 10% since last time, print it too.
       StackDepotStats *stack_depot_stats = StackDepotGetStats();
+      if (stack_depot_stats) {
         if (prev_reported_stack_depot_size * 11 / 10 <
             stack_depot_stats->allocated) {
           Printf("%s: StackDepot: %zd ids; %zdM allocated\n",
                  SanitizerToolName,
                  stack_depot_stats->n_uniq_ids,
                  stack_depot_stats->allocated >> 20);
           prev_reported_stack_depot_size = stack_depot_stats->allocated;
         }
       }
+    }
     // Check RSS against the limit.
     if (hard_rss_limit_mb && hard_rss_limit_mb < current_rss_mb) {
       Report("%s: hard rss limit exhausted (%zdMb vs %zdMb)\n",
              SanitizerToolName, hard_rss_limit_mb, current_rss_mb);
       DumpProcessMap();
       Die();
     }
     if (soft_rss_limit_mb) {