GWP-ASan: Reduce RSS on macOS

Currently we mark unused pages inaccessible but don't release their
memory back to the OS. Release it so that we can make use of a large
quarantine without incurring memory overhead. This showed a ~2-4%
regression on a microbenchmark that looped allocating and
deallocating.

Bug: 912286
Change-Id: Iab5fd441071ef0fc45aa793708b886c7fe864158
Reviewed-on: https://chromium-review.googlesource.com/c/1471331
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632020}

components/gwp_asan/client/guarded_page_allocator_posix.cc

@@ -30,8 +30,12 @@ void GuardedPageAllocator::MarkPageReadWrite(void* ptr) {
 }
 
 void GuardedPageAllocator::MarkPageInaccessible(void* ptr) {
-  int err = mprotect(ptr, state_.page_size, PROT_NONE);
-  PCHECK(err == 0) << "mprotect";
+  // mmap() a PROT_NONE page over the address to release it to the system, if
+  // we used mprotect() here the system would count pages in the quarantine
+  // against the RSS.
+  void* err = mmap(ptr, state_.page_size, PROT_NONE,
+                   MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
+  PCHECK(err == ptr) << "mmap";
 }
 
 }  // namespace internal