tcmalloc: Decrease the kHashbits to 12 effectively restoring the pre-r89459 size

Original CL:

- https://codereview.chromium.org/7200028

  Decrease the kHashbits to 12 effectively restoring the pre-r89459 size
  of the page heap.

  TBR=antonm
  TEST=VM usage drops on the perf bots
  Committed:
  http://src.chromium.org/viewvc/chrome?view=rev&revision=89670

BUG=724399,b:70905156

Change-Id: Id0eedab1c33b53e47970561547f75c440f5d9ea1
Reviewed-on: https://chromium-review.googlesource.com/c/1130808Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597319}

third_party/tcmalloc/chromium/src/packed-cache-inl.h

@@ -143,7 +143,9 @@ class PackedCache {
   // Decrease the size map cache if running in the small memory mode.
   static const int kHashbits = 12;
 #else
-  static const int kHashbits = 16;
+  // We don't want the hash map to occupy 512K memory at Chromium, so
+  // kHashbits is decreased from 16 to 12.
+  static const int kHashbits = 12;
 #endif
   static const int kValuebits = 7;
   // one bit after value bits