Use new tcmalloc on Linux builds by default.

This changes the default version of tcmalloc used by Chrome on Linux
from a modified tcmalloc 2.0 to a modified tcmalloc 2.7.

See discussion at http://shortn/_db7vovHkbm for benchmarks and analysis.

BUG=724399

Change-Id: I75b89fcb0e22c2ca627634a6581c937608f4ce6f
Reviewed-on: https://chromium-review.googlesource.com/c/1278949
Commit-Queue: Will Harris <wfh@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605104}

build/config/allocator.gni

@@ -34,7 +34,7 @@ declare_args() {
   use_partition_alloc = !is_ios
 
   # Use the new tcmalloc. It's relevant only when use_allocator == "tcmalloc".
-  use_new_tcmalloc = false
+  use_new_tcmalloc = is_linux && !is_chromeos
 }
 
 if (is_nacl) {