tcmalloc: Use aggressive decommit by default.

Optimize for memory use instead of performance, to achieve parity with
the old version of tcmalloc.

BUG=724399,b:70905156

Change-Id: I11cf263b8c9595b260bf2046ab9593c2b8f66a00
Reviewed-on: https://chromium-review.googlesource.com/1130812Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586439}

third_party/tcmalloc/chromium/src/static_vars.cc

@@ -93,9 +93,8 @@ void Static::InitStaticVars() {
 
   new (&pageheap_.memory) PageHeap;
 
-  bool aggressive_decommit =
-    tcmalloc::commandlineflags::StringToBool(
-      TCMallocGetenvSafe("TCMALLOC_AGGRESSIVE_DECOMMIT"), false);
+  bool aggressive_decommit = tcmalloc::commandlineflags::StringToBool(
+      TCMallocGetenvSafe("TCMALLOC_AGGRESSIVE_DECOMMIT"), true);
 
   pageheap()->SetAggressiveDecommit(aggressive_decommit);